Just realized: Junpei's plan to go through the #3 door is much more cunning than the game lets on.
So, he has himself, Akane, and Seven authenticate, then pits Santa against the duo of Lotus and Clover.
5+6+7 = 18. 1+8 = 9. If Santa is with them, he has to be left behind at the #9 door anyways, the only option is Junpei + Akane + Seven. If Lotus and Clover are with them, the options for the #9 door are: 1. Junpei + Akane + Seven. 2. Akane + Lotus + Clover. Akane always survives, and there is no way Seven can't power his way to survival.
The whole plan was basically to pit clover+lotus against santa for the #3 door, and ditch the winners at the #9 door, or make a "noble" sacrifice and let the women survive at the end, if Seven was reluctant.