Legally could some billonaire hire a bunch of mercenaries to help out in Ukraine?

Poutine is a war criminal so how the rest of the world responds is likely going to be “in kind”.

While your point has some merit, there is a difference: most of the IB were not professional soldiers (by which I mean had a “career” in regular or mercenary armies) or “security” people before or after Spain. They left civilian life to fight fascism, not just for a paycheque. As Renault remarked to Rick, “the winning side would have paid you much more.”

Civilian life was the Great Depression.

H. Ross Perot helped get EDS employees out of Iran.