Both of your scenarios contemplate this, so I think the incidental details of who get in over them probably don't matter.

HOWEVER, the possible exception is if BYU were to get in over them in scenario 1, I think that could be less motivation to expand, because they would say "if the committee let BYU in without a championship game, then we definitely can get in without a championship game".

However, I HIGHLY doubt scenario 1 would happen. I think if the B12 gets left out again, it's not because a G5 or BYU replaced them. I think last year taught us that if TCU/Baylor couldn't get in with their resumes, no G5 (nor probably BYU) is getting in as the system is constructed. In other words, I don't see BYU getting into the CFP over a B12 school (a one loss B12 school probably gets in over undefeated BYU, and in any case probably both are passed by for one loss teams (and in some cases even a 2 loss team) from SEC, B1G, P12 and probably ACC, where there are championship games)

Interesting question though