The only reason Russia is invading Ukraine is because they feel threatened by the USA and NATO and they need a buffer for their own defense. If USA left NATO and especially if Russia joined after that then Russia would not feel the need to secure a buffer between NATO and Russia.
China probably would not feel as threatened either if the USA were not as involved in Asia as it is currently. I think the world could actually reach a new level of peace if President Trump would withdraw the USA from NATO and isolate the USA from world politics.
No. Like it or not, the world is safer with the US as a policeman than when it isn't. Trump used the US mil to great effect and achieved the closest thing to world peace in my lifetime with it. If we withdraw from the field, China will fill that void.
You won't like those long-term consequences.
No, the world is actually less safe with the USA acting as police. If the USA were not in NATO and actually isolated in the America & Russia and maybe even China were in NATO then there would have not been even any reason for Ukraine war.