. Wesley remains something of a mystery but my unsubstantiated pet theory I'm partial to is that he's still with the Traveler and the time we saw him in Nemesis was the first Undine infiltrator.
Everyone else seems addressed, so i'll nitpick this. In the novels before the Destiny/STO split, Wesley just pops into existence at the wedding, implying that he's now a Q-like being. So he can show up whenever he likes, or do almost anything he wants, excepting the fact that as a Traveler, hes to use his powers to 'observe' rather than effect changes.
Its all so murky because with the exception of Scotty and McCoy, no one from TOS appears or has his/her status mentioned other than in softcanon. And there's also no information on the current lifespan of humans.
There's no problem for Scotty to still be alive i guess, since he'll be 110+ish in 2409. Or should we consider it 2410 now? It doesnt seem to be a problem for humans to live that long.