Russia should try and use diplomatic means and soft power to hang on to their sphere of influence. If they tried to put uncomfortably positioned bases in Mexico, the US has lots of diplomatic and economic leverage to get Mexico to reconsider. We wouldn’t have a war over it.

If the US invaded Canada and Mexico looked to Russia for support in case they're next, I certainly wouldn't blame them.

There is no need for hypotheticals. Russia already has military bases in the middle of Europe and hosts nuclear missiles there. Map: https://img.washingtonpost.com/blogs/worldviews/files/2016/1...

I know this is the worst kind of whataboutism and shouldn't be awarded a response but I'll bite.

Finland joining means it becomes part of a defense pact. It's not like suddenly the "US sphere of influence" expands, or that there are somehow US military bases in Eastern Finland.