There IS going to be a sixth member and we are finding out AT the PPV. They said so on Friday. I'm thinking it will be Jericho somehow managing to get back in and if not, it will probably be a swerve. Darren Young? Please. I see him costing Barrett the title, but no way is he getting into the match. Miz will not insert himself, especially since they've announced that there will be a replacement. For that matter, everyone fantasizing or speculating about Miz cashing in for the WHC, he CANNOT. His contract is for Raw's title, which is the WWE title alone, as he won the Raw MITB match. End of story.
Sheamus likely won't retain, as he will need to be free of the title for HHH's return, and it's incredibly unlikely that the sixth man will be HHH because of that. He might show up to cost him the match, but it's logical to assume that his return would be in a one-on-one after he reappears on Raw or somehow costs Sheamus the title. If for some reason it does end up being HHH, neither of them will win. I certainly don't see their feud involving the WWE title.
I think that the likeliest scenario, based on the five participants we know, is that Orton wins, defends successfully against Sheamus (rematch clause) at HIAC in a grueling match, possibly due to interference from a returning HHH, which then sets up their feud and removes Sheamus from the ring area, THEN Miz cashes in. I doubt highly that the cash-in would be against a heel, so unless they don't plan on letting him cash in for a while, I'm guessing the next champion is a face. If Miz is going to job to DBD, I'd like him to be booked against main event talent on Raw for the next couple of weeks, winning some matches in order to establish his credibility as a worthy champion, cheap cash-in or not.