Hellboy would count as a starter, so MMK would be legal under this scenario.

do we actually have PROOF that hellboy will act as a starter? and what set will it be attached to?

The numbers of sets would be based on dates.

Even if Hellboy is considered a set, it wouldn't bump any of the DC or Marvel sets. [Since there will never be less DC than Marvel sets in a Modern or Silver format].

In the case of other sets that had cards that weren't in the set [the first three] they went with the last set released. I'd guess that would make it Legion. Then again, they might make it Marvel Team-Up, it's closer.

[If nothing else, it could get it's own legal date to rotate in or out of a format.]

Pretty much, Modern is for a year after it first becomes legal, Silver for two years, and Golden past that.