County requests membership
NATO decides to process application
Every member state must ratify the decision That means that every single individual government of NATO members must separately vote on taking in a new member, not just the US Senate. And again this can only occur after the new member voluntarily requests membership.

Joe Biden has been a vocal supporter of NATO, yes. That is largely a question of domestic policy. How do you assert this to be exerting undue influence? For all his talk, if a country does not want to join, they get to decide what is in their best interest, and not join. That is why Finland has not been a member until now.

There was never a formal agreement to halt NATO expansion. As far as I am aware, these claims stem from a verbal agreement with Baker and Gorbachev. If I am mistaken, please direct me to the formal agreement that was made.

Regarding formal agreements, I believe that the Budapest memorandum gave Ukraine assurances against acts of aggression by Russia, the UK, and the US.

Re the invasion coming after Biden changing his mind: Ukraine has had a relationship with NATO since the 90s, and applied for membership in 2008. Russia first instigated this war in 2014 after ukranian protests of their then-pro-russia government. Since then, Ukraine has been more interested in making membership a priority. In 2019 their parliament amended their constitution to prioritize working towards membership. This was all well before bidens presidency, and makes the notion that Biden changing his mind makes Ukraine suddenly seem interested after a long time of being disinterested resulting in the war seem not only incorrect, but nonsensical.

Even ignoring the above deficiencies, the best case argument I see you putting forth is that the US is indeed implicitly responsible, whereas Russia is explicitly responsible.