Yes this is what happens in a double spend scenario
Then you are a small edge case, especially for plausible fork lengths, and even more especially for plausible fork lengths given regular checkpoints (as in Bitcoin and every other reasonable coin). Given the possibility of forks (even normal ones transient ones) you always need to be prepared to reissue your transaction for some reasonable period of time.
The far more likely cases are that: 1) nothing happens, or 2) you simply see the coins back in your wallet and resend them.
Smooth I am sorry you didn't read yesterday's discussion I had with NewLiberty. I refuse to repeat the same discussion again.
Checkpoints are an illusion given a sustained attacker. Once the attackers' chain get mixed up with enough important transactions, you will have users screaming bloody murder if you try to unwind them.
You entirely dismiss the concept of time. Ding dong!
"Hey I sold out of XMR when I saw the attack underway and I got out before the stampede in the price, and I damn well don't agree to clawback of my fiat from Polonoxious to the current miniscule price".
As I said, one way or another, one chain (fork) will survive. Users on the other chain may scream bloody murder, but arguing with math will get you nowhere.
As for what happens with their fiat, that will be between them and their exchange. Exchanges deal with hacks, coin bugs, etc. Its part of the terrain. Some survive it, some don't.