Article 51 applies to states, not continents, but sure some states might have a claim against the US. That doesn’t change anything about the Iran regime’s aggression.
But you brought it up. Nicaragua if we are following the laws of proportionality established by America and Israel in this invasion, Nicaragua alone has the right to level most of the unuted states, and to say nothing of the claims of other south american and middle eastern countries. So personally I'd rather not use the proxy argument unless one is comfortable with the idea of the USA having multiple nukes lobbed at it from all over the world.
Iran presently also isn't trying to Lebensraum "buffer zones" from other countries lands around itself.
Okay, let's imagine that Nicaragua has the right to nuke the US, setting aside the fact that their self-defense justification expired decades ago, and that someone else committing war crimes does not create any legal justification for other war crimes.
I'm still not sure what this has to do with Iranian aggression?
Israel and America are the ones who directly invaded Iran during "negotiations". If you want to get at proxies, then again as I said, I would support arming Nicaragua, Venezuela, Cuba, etc with nukes and having them launch them at the USA. If not, then Israel and USA attacked first, Iran hasn't been directly attacking them during this time.
If we correct your argument so it's actually based on international law, it seems to boil down to "we have to ignore proxy warfare, otherwise Nicaragua might have historically (several decades ago) had a right to attack US military assets." That hardly seems like a reason to deny reality and ignore proxy warfare.
Iran presently also isn't trying to Lebensraum "buffer zones" from other countries lands around itself.