...For that to work both sides need credible security guarantees from strong and stable third parties. That rules out the USA, NATO and the EU unfortunately. ...
It's the "security guarantees" that I can't see working. Ukraine already has the Budapest Memorandum and whilst obvious Russia hasn't compiled, neither has the US. So they already have guarantees that have proven totally ineffective with one signatory even switching to support the aggressor.
So what can they be provided that they can trust. eg if Trump said "US will send vast numbers of troops to fight off any aggressor guaranteed" could they trust Trump won't change his mind after 10 mins?
What country is going to put in their own nationals (military) to exchange fire (and deaths) with Russia when they decide on Phase III? Would the UK or would be be prevaricating and our leaders worried about domestic challenges to their leadership ...
Trump has clearly demonstrated to the world how he discards allies on no notice.
Even promises of vast munitions in the event of Putin restarting might just turn into "we don't have the stocks without depleting our own", or they just don't arrive as internal domestic squabbles are played out.