i think a lot of points made are not very good proof just some half-connections. The idea that hes from the ancient dark tribe is a good idea, but you are all forgetting that majora was not from that dark tribe, majora was from a race that was around well before the dark tribe existed, and majoras people celebrated the mask ritual every year. majora was ridiculed for having a mask that was different and all of his dark emotion and hate went into the mask, making it evil. the mask was then used in hexing rituals much later in time by that dark tribe you are talking about. so the mask salesmen would more likely have been a descendant from the same group of people as majora, not the dark tribe. This is especially obvious because of the children in the moon looking like the mask salesman. the main point that i am making here is that he is clearly not from the dark tribe, but more likely from the same group of people that majora belonged to.
the one thing that is clear is that the mask salesman is very mysterious, seemingly all-knowing, and has the ability to travel between parallel universes/dimensions. the happy mask salesman is most likely a very powerful entity. the happy mask salesman could be a person from the same race as majora who saw what that mask did to his people and could be trying to prevent it from happening again. he may even be majora himself. majora doesnt have to be evil. it was his mask that was evil. he couldve recieved power from the mask to live forever and feels guilty of what his mask did.
i dont really think that the happy mask salesman is evil, he got mugged by the skull kid, and clearly stated he was trying to stop the mask from getting into the wrong hands because it was an evil mask. hes more of a guardian and a protector, a god-like figure almost.
this is a very funny little side-fact which is almost definitely a coincidence, but technically, he is literally "guiding the chosen hero of the goddess
from the edge of time/universe" (sound familiar?) because termina (root of that word means end hinthint) is an alternate universe/dimension connected to hyrule by that hole in the lost woods. that rift between the clock tower where link enters and the hole in the lost woods that link falls down into could technically be considered the edge of time/ the end of the universe for both realms. i dont actually think this is the case but its kindve funny to see how it fits that scenario almost perfectly. if only nintendo thought out that much of the story (which they clearly didnt).
either way i do think that the happy mask salesman is a nearly all-knowing entity, and good point about connecting him to majoras people through the children inside of the moon.