a surrender condition that lets Russia retain all the land they’ve taken
A while ago there was a prediction about the true intentions of the russia negotiating: they want to stop Ukraine from succeeding as a state, since that would be a bad omen for a neighbouring kleptocracy.
If this prediction comes true the outcome you describe is actually very unlikely, because the russia would rather continue fighting just to keep the region in shambles.
On the short term: maybe
On the long term: makes absolutely no difference. Some of the soot from those fires might actually settle in the soil and stay there for 100 - 1000 years, which is probably the least harmful way petro chems could be converted. The portion that would have been converted to plastics and rubber might be a grey zone, but not a large one, since a large portion of that is burned at the end of its useful life.