While the OP concentrated on the Western front, how about the Eastern? Even beyond advise along the lines of “Watch out for that Lenin guy,” could things have been turned around at Tannenburg? And how much of a difference would it have made if you could?
Creeping barrages were certainly used in 1916 on the Somme - possibly earlier (I’m working from memory). One of the major problems with the creeping barrage in WW1 has already been alluded to - communications. With no way to feeding back to the gunners the speed of the advance the tendency was for the barrage to lose contact with the advancing infantry. The rate of advance of the barrage had to be preplanned at - say - 50 yards per minute but the infantry were frequently held up by barbed wire or unsupressed enemy fire with the result that the barrage moved away from the infantry allowing the enemy to come up out of their dugouts. And, yes, accuracy had certainly improved by 1918. The guns were better, the shells were more reliable, and the gunners were much better.
As to confidence, psychologically it is one thing to risk enemy fire it is quite another to risk being blown up by your own side. The training had to drum into the plough-boys and factory workers that they were a lot safer staying close to the back edge of the barrage - risking a shell falling short - than they were holding back away from the line of exploding shells in front of them with the strong chance of the enemy manning their trenches before they could be taken.
I don’t think there was ever a period in WW1 when the German effort was overwhelmingly directed at the Eastern Front - the Western powers were just too strong to allow the Germans to turn their back on them whether in 1915, 16, or 17. For most of the war the bulk of the German army was in the west.
As to providing support to Russia, well Britain and France were hard pressed to meet their own needs and until 1917 the US was not even a co-belligerent - greatly objecting to Britain trying to enforce a blockade on Germany! It is difficult to see how a different C-in-C could have done anything to change sentiment in the States and/or help Russia.
A previous thread covering much the same ground: WW1 refought 1918 style.
Essentially, cut out 3 years of learning curve and go with what had proven to be somewhat more successful by 1918: give up trying to make the “big breakthrough” that would never come, and switch to a “bite and hold” strategy- which while still a slow frustrating slog could actually produce results.
ETA: as said upthread
The Thompson gun would have been the American version, but it came too late. Not sure how much increased priority could have sped it’s deployment. While the sub-machine gun was ideal for trench warfare, the “Trench Gun” shotgun was also used to good effect.
I suspect no as modern battle tanks and infantry fighting vehicles with reactive armor and gas turbine engines to smash through fortifications, helicopters that can deposit troops behind the trenches, drones to monitor troop movements, precision laser guided bombs and the aircraft to carry them to blast bunkers, Kevlar body armor to reduce casualties, not to mention modern logistics command and control systems didn’t exist.
Although I wonder why they didn’t use something more along the lines of medieval siege tactics? For example, digging trenches zig-zagging towards the enemies trenches until they connect and then start attacking.
High explosive shells pretty much would have rendered medieval style tactics futile and deadly to anyone trying to use them. They only worked because of the fairly anemic artillery available during that period…earthworks could defeat even the direct fire artillery available and allowed siege lines to get close enough to fortifications that you could bring your own artillery to bear (or wolf or lion).
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My point is either of us could do this, and as far as I know neither of us has ever held a commission as an officer, much less the rank of general. A general from 2014 replacing a general from 1914 isn’t going to change anything that transplanting a layperson with an education in the military history of WWI until the present day with a man in the street in 1914 couldn’t change as well. Knowledge of modern military tactics and strategy isn’t going to be of any use to this time travelling general since they rely upon a means of communication that didn’t exist. Send 10,000 portable radios back in time and you’d make a much bigger impact than replacing a general of two from 1914 with ones from 2014.
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I agree…if we are talking about putting a modern general in charge of the already trained and equipped allied armies of 1914 then their impact would be minimal at best (this assumes they could simply order the command structure and have their orders carried out to even get to ‘minimal’).
The advantages of being able to instantly radio aerial reports was obvious, and they worked on inventing air-to-ground radio: Air-to-ground communication - Wikipedia. But although radio technology advanced rapidly, it couldn’t be perfected before the end of the war.
Could balloons have been used more effectively for communications?
I can’t help but picture some future time travelers telling each other, “the best way to avoid the grievances that caused so much bloodshed to persist through WWIII and WWIV would be to go back and assassinate Franz Ferdinand in the first place.” 
How timely, as I’ve been watching the BBC series “The Great War”. The Germans had really good artillery and for a long time, the Allies were really short on artillery shells. So, a modern replacement general for the allies should have upped production of artillery/shells and promoted aerial bombardment of the Krupp works. Which would have required a lot of ASAP development of long range bombers.
A modern general might have championed “counter battery fire” which could have been effective. There were rudimentary systems of locating enemy artillery available during The War to End All Wars, but they could have been tweaked and used more and more efficiently. Lack of communications could have been the problem.
The tanks of the era weren’t the tanks of WWII. They were just slow moving machine gun nests that could move over barbed wire. They were operated by the mechanics and infantry. It wasn’t until early WWII when tanks were more reliable and advanced and displaced cavalrymen took over the branch and applied cavalry tactics to Armor did they really become effective.
A modern general who’d require a minimum 4:1 numerical advantage and total air supremacy before advancing would be problematic.
Somebody who was aware that munitions could be targeted with laser precision in a couple of decades but needed to use dispatch riders and carrier pigeons to marshall their forces along a 460 mile front would likely have found the situation as challenging as the generals from previous wars…
I would think a modern general wouldn’t throw repeated advances against machine guns and hence the war would have gone defensive much earlier. Concur they would have sought for the deployment tanks as a priority but to break the trench stalemate before they had reliable mobile armored units, would they have used chemical weapons earlier?
My impression from reading about WW1 is that gas only worked well in the beginning, when it was a surprise. After both sides were prepared for it, it simply added casualties without offering any real advantage.
… wasn’t adding casualties sort of the point ? 
Balloons were widely used for artillery spotting. The problem was that they were very large, very immobile and very flammable, because they were filled with hydrogen rather than helium.
So they were basically target practice for the flying circus, and not a very sought out assignment.
The other issue was that they were kinda hard to miss, so trench boys who saw a bunch of balloons rising up above the enemy lines knew they were in for a shelling and immediately took cover…
They had a series of pre-timed lifts onto the next objective, which was all the inexperienced artillery were capable of doing at that time. To do a proper creeping barrage took a lot of experience.
What you’d have wouldn’t be a modern general, or any general for that matter. The 3:1 rule of thumb is frequently misunderstood to mean what you imply above; that isn’t what it means at all. What it means is that the attacker needs to amass a 3:1 advantage in combat power at the decisive point to assure a successful advance, not a 3:1 overall superiority in raw numbers along the entire length of the front. This figure is further subject to force multiplying effects; against a force that is deeply entrenched in a defense in depth the figure goes up to 9:1. It is little wonder that successful advances against the trench lines couldn’t be maintained and the front lines rarely moved more than a couple of miles in either direction. The defender could much more readily reinforce weakened points in the line than the attacker could exploit them even before the problems of communications the attacker had are taken into account.
Why wouldn’t they have? Sometimes the only option is a frontal assault against a prepared opponent who is your technological equal; I’ve no doubt a modern general would do so as willingly as any general throughout history has. Generals didn’t throw their troops into repeated frontal assaults in WWI because they were sadists; they did it because it was the only thing they could do.
They’d be waiting until the 1930s for reliable mobile armored units. Ranger Jeff described them well “slow moving machine gun nests that could move over barbed wire”. WWI tanks were often slower than the walking pace of a foot soldier and were anything but reliable and mobile. They weren’t going to achieve the kinds of armored breakthroughs and exploitations into the enemy rear areas of the kind that happened in WWII. Not even taking their slow speed into account, they would almost all have suffered a mechanical breakdown before running out of their first tank of gas - especially with the moonscape of terrain that no-man’s land and the trench lines had become.
Contrary to popular opinion the troops were hardly ever ordered to do this. The fact that sometimes things went wrong and they ended up doing it anyway is another matter.
As was said, the biggest problem was communication. During an advance there was no way for headquarters to communicate with the advancing troops. There was no way for blocked troops to tell headquarters that they were retreating, or successful advances to request additional troops to exploit the breakthrough.
As for "wait until you’ve got an overwhelming advantage, and only then order the charge across no-man’s-land, well, that’s what they tried to do. But their estimates of the relative strength of the enemy compared to your side was often incorrect. And so you build up an overwhelming force, gamble everything on a last irresistible assault…and the assault earns you a few miles before stalling.
I will submit that sending a modern German general back to 1914 might easily have won the war by Christmas, simply by abandoning Prussia to the Russians, hollowing out the left and center, and putting everything possible into the right wing according to the original Schlieffen plan. Also maybe don’t massacre the Belgians so much? You’re going to have to incorporate them into the Empire after the war, so let’s get started now. Of course, this requires you to get Kaiser Bill to listen to you and not interfere.
Get your troops to Paris, and the war is over in a few months. There’s your tactics–or rather strategy–to shorten the war. Or does it only count if the war ends in an Allied victory?
The problem with the Schlieffen plan is that if for any reason it doesn’t work, then you’re screwed, which is what happened. A quick victory was Germany’s only hope for a mobile war in the west, and when it didn’t happen the trench stalemate resulted. If you’re going to postulate a different strategy instead of different tactics, then in hindsight Germany could have simply fortified it’s western border, and let the French batter themselves to pieces on it until they decided supporting Russia just wasn’t worth it. Furthermore if there was no invasion of Belgium then Britain might not have committed to the war. Given that IRL Germany finally defeated Russia, only too late to resist the Allied advance, it would seem Germany would have done even better had it simply held on in the west, without provoking the British into the war and giving France an “out” to reach a separate truce. As it was, because Germany had advanced onto French soil, the French couldn’t and wouldn’t stop fighting short of absolute collapse.