I think the words of Martin Kimani (Kenya's UN ambassador) prior to the invasion really put this in perspective, and clearly show Russia's actions go against the spirit of the United Nations.
Hmm... I wonder if before the invasion, some third-party nation at the UN had said "These are serious and concerning allegations of what is happening inside Ukraine. But in the absence of evidence, I propose UN peacekeepers and observers (from a neutral nation) are sent to the Donbas to investigate ceasefire violations and see what is going on, reporting back in 30 days."
I doubt it would have changed anything, and it had the danger of giving a smidge of credence to Putin's lies, but I think it would have made Putin's justifications all the more flimsy when he rejected it.