Regarding the last remaining faction and how it will be determined, I think this question is still open. I agree with dwp; the player will, in any case, plan which faction they will leave for the first game of the final series. Therefore, having two factions remaining, the participant will most likely know exactly which faction they will save for last and which they will use in the last game of the group stage. And if this last game doesn't happen (not due to the participant's fault, by the way), then I don't think it's fair to deny them the right to choose!
Here's a suggestion: the participant who is awarded an automatic victory must, in any case, declare their faction for this "game," which will then be removed from their list. Given that all of this will happen in advance, all participants will approach the games in the final round with only one faction remaining in their lists.