Nobody cares about Russian nuclear threats when Russia is still following all best practices in telling the world where their nukes are going and when. Either way, China doesn’t support Russia using their nukes, so Russia won’t in order to keep their support.
The truth is nobody besides the most hawkish wants Ukraine joining NATO right now, because that would grant them access to article 5 and thus boots on the ground from every other NATO state. Ukraine’ll almost certainly be invited into NATO once either they regain Crimea + other occupied lands from Russia or if they renounce their claim to those territories.
I’m pretty sure nothing in NATO’s articles explicitly requires a joining state to have their territory secured, at the very least article 10 says nothing about it. I believe it’s hypothetically possible, if consensus exists among each member state, for them to grant membership to Ukraine, for Ukraine to immediately call on article 5, and for the United States and everyone else to militarily intervene immediately. It’s just that in reality no one wants that to happen.