This war is about the long term, not the short term. Whoever wins will have control over these disputed areas for decades. For Ukrainian particular, the combination of surrendering a bunch of its territory and not being permitted to join a defensive military alliance means it is likely just a matter of time until Russia rearms, retools, and moves to gobble up the rest of the country. As Putin himself has stated publicly, he does not believe Ukraine is even a legitimate country or has a right to exist. So the Ukrainians uses as ultimately a war for national survival.
In other words, Ukraine rightly does not believe that Putin would abide by any agreements that were reached. Any security guarantees made by Russia would not be worth the paper on which they were printed. Given that, they have no real incentive to agree to Russian terms.
I didn't even read all that you typed here, but what is your opinion of Northern Ireland? Northern Ireland is the very same as the Ukraine - Russia question.
I do not see how Ukraine is the exact same thing as Northern ireland.