P2:
The adjective belligerent is "hostile and aggressive". A party defending another's right to exist in the face of an aggressor nation conducting an illegal war "Special Military Operation" is not a belligerent. If you defend your family from an intruder, do you refer to yourself as a belligerent?
The noun belligerent, which you are using is "a nation or person engaged in war or conflict, as recognized by international law." As Russia went out of their way to commit criminal acts, and has not declared a war, I think using the term belligerent while correct in a war is somewhat inappropriate for a country undertaking self defence against a criminal act, and that would include any country assisting in the collective defence of the victim. "Belligerent" has connotations of aggression in all uses.
If Russia can call a criminal act an SMO, I see no reason not to call Ukraine a defender and those countries that support Ukraine co defenders.
The UN Charter permits collective defence, what that may mean to NATO is anyones guess, but Poland and presumably Estonia if they get involved within Ukraine would make that an interesting question for Vlad to sleep on. An attack of Belarus itself is high stakes, and would come with baggage. assisting Ukraine repeal the uninvited crime would be consistent with the UN Charter and a response directly against the country concerned would be a guessing game for Vlad as to where NATO may draw a line. So far he has been drawing blanks on his assumptions of world response to his summer vacation plans.
Should you wish to double cross this bride, you can PM me and we can take the discussion off line. Any assumption that NATO may not not respond assists Vlad, and I don't consider that appropriate in light of his barbaric actions with Ukraine and even his own citizens.
I think you've now sussed out the point. If a party elects to become a co-belligerent then NATO Art 5 no longer applies.
I would like to hope UN applies, but that hope died stillborn back in 1945 when circumstances meant that the same problems as bedevilled the League Of Nations were baked in.