>if Putin never would have allowed an independent Ukraine, why would NATO reject his demands to reject Ukraine's NATO application?

Euromaidan happened 2013. Putin already lost the Ukraine back then. By annexing the Crimea and sending Russian troops now he is taking the Ukraine back.

I may be mistaken but I think whether the Ukraine was actually joining NATO or not wasn't really relevant in this conflict IMHO.

>If Russia was going to invade anyway, why not at least try to de-escalate and negotiate for a sovereign Ukraine?

For the couples months there were negotiations. Was there a possibility to avoid the current conflict?

I think unless you're the U.S./Russian diplomat it's impossible to answer this question.