'How do you reconcile Trump saying he'd use nuclear weapons in Europe and him leaving all doors open for Putin to do what he pleases?'
Trump always says crazy things because as the legendary Stump for Trump sisters so often say "he is a baller, a shot caller", he does it for bargaining positions and for deal making so that his opponents don't know what to expect and cant read his true intentions.
He is not holding doors open for Putin, he is a dealmaker who recognises reality and doesn't want world war (like the hawks and their moneyed backers want) but instead wants to make deals with Putin for the benefit of the United States and its people.
'The US and Russia have backed different sides before without anyone going to WW3, even at riskier times during the cold war. '
Yes, but now the situation is far more precarious because the moneyed backers are losing control of the economy, the debt burden is huge and the derivatives' position is enormous, which is why some are edging their war hawks to escalate the situation. Trump knows all this and intends to stop it and make America great again by ending the moneyed backers globalist game and doing deals with Putin and China and whoever else.
'Putin is not stupid enough to want WW3, either.'
Of course Putin doesn't want world war, it is the war hawks and their moneyed controllers who want it.
' With Trump there'd be none, or he'd directly threaten with the nukes (if he could find them).'
You understiate Trump. COntrary to what the BBC and Channel 4 says about Trump, the whole world will respect him and America when he takes over. Trump is not mucking about, he thrashed the Republican Establishment and world leaders are all very respectful of him now that his victory is close and after he wins, the grovelling will be like we have never seen before.