So if Trump becomes president and evidence is found that he had sex with a minor (before he became president), it would legally no be possible to impeach him for that?
It's uncertain. Technically, there's no limit on impeachment other than Congressional will. If they're actually willing to go through with it, they could do it. It'd cause a legal clusterfuck, and Trump being Trump he'd probably refuse to surrender, but it's not impossible.
The thing is that there's a certain lack of precedent here. Clinton wasn't convicted, Nixon resigned before he could be, and nobody really pays attention to the impeachments before them.
Can a president be jailed?
Legally uncertain, practically no. They're too important, and you'd also have to get the cooperation of the Secret Service. There was once an incident after, iirc, the Reagan assassination attempt, where his security detail nearly gunned down a bunch of local cops trying to bust into the hospital he was in.
So long as this country doesn't do something stupid like heavily restrict immigration.
In all honesty? I don't buy that it's possible, even if Trump were to be elected. Immigration on both sides of the law is the lifeblood of the US. The runner up in sheer numbers is Russia, and theirs is entirely different in nature to ours. Trump would hurt a lot of individuals but I doubt he, or any other person, could ever put a dent in immigration that wasn't caused by greater trends.
Maybe, no matter who wins, we just need to split the country. There are such firm political divides between red regions and blue that having a single unified government just doesn't make sense. The difficulty, of course, will be in resettling displaced people, but perhaps we can do home-swapping. If you're a liberal in rural Alabama (somehow,) you can switch places with a rural conservative in California.
Jesusland and Liberalia have been proposed before, but maybe the country is ready for it now?
I know you're not serious, but god this attitude pisses me off. The country isn't firmly divided into Enlightened Liberal Utopia and Hurr Durr Jesusland, the divides are enforced through lack of campaigning and gerrymandering. Even in the reddest and bluest parts of the country there are firm cores from the other side.