Any formal treaty between Ukraine and Russia would have to be signed by the international recognised Ukranian governmet to be seen as legitimate by international law:
That's why I said continuing the war until Zelensky agrees to an unconditional surrender. You can't win a war by repeated refusing what your opponent is offering you as you're losing your entire country. He could flee the country and run a government in exile from America I guess. He'll be in good company with the current president of Afghanistan.
Not currently with war between the two nations, no

But later down the line after an undetermined period of occupation, puppet regime or even annexation. It happened to the Soviet Union and even threatens Russia internally today. That's the problem with empires.
I thought you were referring to a neighbouring country like Poland being hostile to Russia. If that's what you meant, now's the time to fight. They won't get a better chance in the future.
If you meant to say the population of whatever part of the Ukraine Russia occupies may become hostile and starts an insurgency, I agree it's possible. I think they'll go the way Germany did post-WW2 rather than the Afghanistan or Iraq. Being used as a pawn by NATO for years before being abandoned when they actually needed them is going to hurt their pride a lot.
If still they really hate Russia I think they'll just move to Germany as refugees. But we'll have to see.
Just like Poland, Finland and others which were all part of Russia or the Soviet Union before. Most of them are not that fond of becoming part of it again and are not exactly friendly towards Russia today. Historical animosity have a tendency to linger for a long time.
Finland has pretty good relations with Russia since WW2. I have no idea on whether Finns have any historical animosity towards Russia, but their government has done an excellent job of maintaining neutrality and not letting its country being used as an international battleground.
Countries do eventually forgive and forget. Most Europeans seem to have forgiven Germany for the actions of the Nazis. Except the English of course.
Germany and Japan became friendly because both actually benefitted by the actions of the conquerors long term. Carrot, not more stick. Had they not, both would be looking for getting even today.
LOL. Did you hear about the Soviet occupation of Berlin? Germany lost a portion of their territory, was split in half for 40 years and had ethnic Germans repatriated. I do agree that Japan largely got the carrot, compared to Germany they got off virtually unpunished.