Don’t all those affect how far the OP’s steel missile will fly?
Only to the extent that a steel missile isn’t completely ballistic. But it’d be pretty close.
Well, all those things are a standard part of ballistic calculations for projectiles fired from rifles, pistols, cannon, etc. Seems like the same should apply here.
"Takanashi’s coach “said her suit was supposedly too big around the thighs, even though she wore it in the women’s normal hill event on Saturday,” Japan’s NHK reported. “He added that the extreme dry weather may have affected her body’s moisture content.”
How low must the ambient humidity go to cause major bodily shrinkage? Could be a useful trick for dieters planning their weigh-ins. 
Not enough to matter. A long, slender shape made out of a material eight times as dense as the human body and fitted with small fins at the rear to keep the pointy nose pointed forward will experience negligible aerodynamic drag losses over the course of its flight compared to the kinetic+potential energy it has at launch.
We’re talking about a very aerodynamic missile weighing 150 pounds and traveling only about 70 MPH at launch. You can calculate the drag: for a missile with a diameter of 4 inches and a length of about four feet shaped like a “streamlined body” (see chart at link), the drag coefficient is 0.04. At 70 MPH, that works out to a total drag force of just over half an ounce. This is vanishingly small compared to its 150-pound heft, and in the end, we’d be satisfied with a model of ballistic flight that predicts the impact point with an accuracy of a few inches.
When speaking of a bullet fired from a gun, we are dealing with a projectile that weighs a fraction of an ounce and travels at speeds of 1000-3000 MPH. Drag force scales with the square of velocity, so the drag force (both axial and lateral) on a flying bullet will be very large compared to its weight. We are striving for a targeting accuracy of a fraction of an inch over distances as much as a mile, so yes, under these circumstances aerodynamic drag forces (axial and lateral) must be considered in order to achieve that kind of accuracy.
Your points are valid. But there’s also the point that the shape of the hill is such that the projectile will be traveling roughly parallel to it - meaning that small aerodynamic effects may meaningfully change the distance covered.