If Ukraine just allowed Russia to overtake their government.... ?

1) Ukraine and Georgia comes to mind if you wish recent examples...

2) Ukraine's government's goal is to remain a country with in the borders that are recognized internationally; but by your standards the Polish should simply rolled over when Nazi Germany invaded since there was no reason to fight.

3) If Russia offered to pull out from all Ukrainian soil there would already be peace as that was the offer Ukraine brought to the table, Russia refused. Russia wants a puppet state which will never be acceptable to Ukrainian people (hence the previous puppet being kicked out in the first place). Russia can try to occupy Ukraine or install a new puppet there but neither of them will work (to costly in $ and life and it simply will not last as the Ukrainian military will not support the new leader). The end result now no matter what Russia does is that Ukraine will pivot even harder towards Europe and Nato because of Putin's actions, and even Putin can see that; so why would Ukraine give it all up to become a Putin slave state again?

4) Yes; the Ukraine government wants to actually not be sent to a Siberian gulag which is the current plan by Putin by all accounts due to the "Nazi leadership" by a jew, or have you missed the confused and contradicting statements Putin and his band of sycophants have been putting out? They win by remaining in place as Putin can't win a short, let alone long, term war.

Here's the simple status, Putin has fked up and is now in a lose / lose situation. No matter what outcome (full occupation, partial occupation, bomb it down etc.) will end up with Ukraine outside the Russian sphere of influence. Other countries he'd like to try to control has taken notice of Ukraine and are moving even more rapdily towards the west as well. He's costing Russia soft and hard power by engaging in a war he can't win for a price he's already lost. Putin will go down in history as one of the worst leaders of Russia and even if they flatten Ukraine before they leave the west will help build Ukraine up again, which is more than Putin will be able to say about Russia who will turn into an African style country exporting raw material to China.

Russia was in the same position in Afghanistan; they greatly outnumbered on any number you wished to use against the local warlords and a decade later left with the tail between their legs and 15k souls less. If you want something more recent then look at Chechnya and what happened. The same applies in Ukraine; yes they can for sure dominate it military and place soldiers in every city (but once again they need to guard over 40 million people and can only put down 100k or so soldiers total) but they can never win. Because while yes they will hold the cities (and spending a fortune on occupation) the Ukrainian people will never accept them, or what ever puppet dictator they wish to prop up. There will be constant ambushes, bombs etc. against the Russian soldiers there, the soldiers there in turn will ask wtf are we doing here with low morale. And if they pull their military out then there will directly be an revolution kicking out the puppet dictator forcing them to send the forces back in again. And when ever they leave then Ukraine will directly pivot to the West and Nato again putting them outside the Russian sphere of influence. Hence; Russian can't win; all they can do is stall at best and that stalling is going to be very expensive both in pure money (occupations are not cheap), people (soldiers killed) as well as morale and becomes a black hole for no return.

So TLDR, can Russia flatten Ukraine completly before pulling out? Yes, no one disputes that but that's not considered a victory for Putin; Putin wants Ukraine to not pivot to EU and Nato and that's completely out the window no matter what they try to do. On top of that Putin has as well accelerated other countries he wished to remain in his sphere of influence to run the other direction; Putin wanted to be seen as the person who regained Russia's old glories and land and instead he'll be seen as the biggest failure of a Russian leader because not even red nose Yeltsin lost as much...