If there are multiple sons the younger ones typically did not receive any of the lands or title, I actually saw the Queen of England in Wyoming as a child when she came to visit some of the descendants of this practice.
I found a news story from that visit that explains the practice.
Ever watch/read James Michener’s Centennial? That was how I found out about them.
Otherwise you end up like the French aristocracy, with Counts and Vicomptes all over the place with no money.
Younger sons typically went into the Church or the military or - as you note - to the colonies or tried to marry heiresses. Or killed off their older brothers.
Or waited dot them to die from alcoholism or syphilis.
Eldest sons also often went into the Army before they inherited. They also were expected to at least assist younger brothers. For example the second Earl of Mornington, who was Governor General of India appointed his younger brother Arthur to his staff. Always wondered what became of that lad.
Arthur, late in his life, was very influential in the early development of the basic frameworks that underpin some of the simpler variants of… shoots self
NM
For those playing along at home, the second Earl of Mornington (Richard Wesley) is better known as the Marquess Wellesley, who served in the office that eventually would be called Viceroy of India.
Richard’s younger brother Arthur went into the army and served in a series of military and political offices. He is perhaps most famous for being portrayed by the brilliant character actor Hugh Fraser in the British TV series Sharpe, starring Sean “Ned 'Boromir 'Stark” Bean.
Arthur also appeared as a character in an episode of Blackadder, portrayed again by a brilliant actor, Stephen Fry.
Surely, Arthur Wellesley could not have conceived of such honours during his lifetime.
Sadly (for you), there is currently an Earl of Suffolk (the 21st Earl of the 4th creation of the title, who is also 14th Earl of Berkshire), and the Brits don’t double up on titles. There won’t be another Duke of Suffolk while the line of Earls endures.
Nobility with no money looking to marry heiresses basically summarizes all of Trollope’s novels, from 150 years ago. 
One of my favourite Earldoms too - at least, the 20th Earl of Suffolk is, because he had a crazy, crazy life.
Entirely up to the monarch of the day, if it’s to do with giving the spare an extra title. Victoria’s children included Dukes of Albany and Connaught, both of which died out.
Unlikely to be an issue in relation to honouring ordinary citizens. The last time the question was raised (possibly) was in relation to Winston Churchill on his retirement as PM. The story goes that he declined, possibly because he wanted to stay in the House of Commons as long as possible (traditionally, a retiring PM used to be made an Earl, but that hasn’t happened since Eden and Macmillan, the immediate successors to Churchill).
Whether at some point a future PM might feel inclined to suggest that doling out additional titles within the royal family looked a bit too Ruritanian for modern times, I don’t know. Probably not, they mostly have other things to do with their time.
There’s a difference between using the name of a foreign place (in Alexander’s case, that was because of the WW2 campaign he was most famous for) and giving a title to a foreign citizen (has been done, for some bizarre reason, Thatcher nominated Caspar Weinberger for a knighthood). That would depend on what the foreign government in question might have to say, and whether anyone in London cared if they took offence.
Well Alexander (and Fraser, Montgomery and Moutbatten)had won great victories at the places named after them. Alexander oversaw a mass capitulation at Tunis.
So when the British I Corps occupies California and puts down whatever undesirable elements there are there, then I suppose its commander Lt Gen Bufton-Tufton might be granted such an honour. 
Presumably if Neil Armstrong had been a British citizen*; he might have gotten Baron Armstong of Mare Tranquilis
(He was quite close IIRC, being the son of immigrants)
There can be a process of negotiation over the exact details of the title in cases like that, with the recipient having a certain amount of choice (provided there’s no room for confusion with someone else, or something not sufficiently worthy-sounding about it). But my guess is he’d have got a knighthood (though they’d probably look for an order with some extra ranks in it*, given the extra-extraordinary achievement involved): a peerage would be more likely if he ended up running the organisation.
*The Order of St Michael and St George is famous for this: it’s used to honour great services in diplomatic and Commonwealth affairs, and recipients get to use different letters after their name, according to the rank involved. So a Commander gets CMG, a Knight KCMG and the greatest panjandrums who become Knight Commander get KCMG - known in the Civil Service as “Call Me God”, “Kindly Call Me God” and “God Calls Me God”, respectively.
The usual. Dabbled a bit in politics.
As did Lord Randolph Churchill, third son of the 7th Duke of Marlborough. Go into politics, rise high but spectacularly crash and burn, die young of either syphilis or a brain tumor, and have a son who worships you despite your ignoring him follow your path to even greater heights.
We’re talking about the creation of dukedoms rather than duchies, really. The title Duke of Cambridge was dusted off for Prince William when he got married, but there’s no Duchy of Cambridge per se. William doesn’t get any land or special rights and responsibilities over Cambridge or its surrounding area.
Prince Charles, on the other hand, is Duke of Cornwall, and the Duchy of Cornwall still exists and owns a fair amount of land in that area. But again, the title doesn’t confer any special legal status on Charles as far as I know. I suppose there may be archaic by-laws about the right to herd sheep on Bodmin Moor or whatever.