The US would not retaliate militarily if Russia used nuclear weapons against Ukraine. That is not part of US nuclear doctrine. Ukraine is not a treaty ally.
>Seek immediate Security Council action to provide assistance to Belarus, Kazakhstan and Ukraine if they "should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used".
I don't know exactly what "Seek immediate Security Council action" means. What if the US seeks it and doesn't find it? Is that in compliance with the memorandum?
#2 Refrain from the threat or the use of force against Belarus, Kazakhstan and Ukraine.
Both violated in the Krimean annexion 2014. Since Russia could easily violate this treaty, they could easily invade Ukraine now. And they got no weapons from signatories, only a few from the Baltics.
Treaties with these signatories are not worth the paper, otherwise Ukraine would have kept the nukes, and would have somehow got the codes also eventually.