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.
They can't even join NATO until then. You have to have control over your border and it can't be disputed. If they gave up that territory then it would be a question of whether countries would want to put boots on the ground but until then it's a moot point.
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.
I undrstand your reasoning. But you can’t ignore that there are a number of standards, not only security-related but also legal and political, that Ukraine must meet in order to be admitted to the defense alliance. It would likely require a series of reform requirements that need to be fulfilled.