Well, the theme of Night Of Champions is that all titles are on the line, so that's a big reason Seth is being forced to defend both titles at the PPV. I wouldn't expect him to start defending both championships on every PPV from now on, but I still can't see him carrying both titles for very long, since he'd still be obligated to defend both at some point and I don't see that as an act that carries on for very long.
I actually think Hell In A Cell could end up being a triple-threat between Rollins, Cena, and Sting for only the WWE Championship, regardless of who walks out with the US Title at the end of NOC, and the match concludes with Rollins pinning Sting to get his championship back. Not sure they'll go that route, since Rollins is a possible candidate for breaking Punk's 434 day title reign and dropping the title and then winning it back the next month anyway would disrupt that in a big way (as in, he's already about halfway there to matching and surpassing Punk's record), but it's an idea.
Another possible idea for Hell In A Cell is that the triple-threat match between Rollins, Cena, and Sting is actually for both championships, but it's dependent entirely on who pins which champion. Let's say this Sunday that Cena wins the US Championship and Sting wins the WWE Championship. At HIAC in the triple-threat, the stipulation is that if someone pins Cena to win the match, they become the United States Champion (and if Sting makes this pinfall, he retains the WWE Title as well, of course), or if someone pins Sting to win the match, they become the WWE Champion (and if Cena makes this pinfall, then he retains the US Title, too.) And the match concludes with Seth pinning Sting to become the WWE Champion again while Cena gets to walk away still holding the US strap and without having to take another direct pinfall loss.
Or, more predictably, they just go with Sheamus vs Sting (if Sheamus does cash in on Sting, should he win the title) for the WWE Championship and Cena vs Rollins for the United States Championship in separate matches.
The dialogue between the Authority and Rollins seem to be heading in the direction of a face turn, but I hope it dies down.
Should HHH and Rollins square off, the former will insist on looking relatively strong against the latter heading into Wrestlemania.
If Rollins turned face anytime before Mania, the plethora of heels for the main event match at Mania will be dim.
For this reason, I believe the writing team ought to cut back on the tension they seem to be creating between Rollins and the Authority.
As for your theories, they seem pretty sound though HHH is a huge proponent of lengthy title reigns (unless you're an absurdly popular 'B+ player').
Of course, Sting's loss at Mania and subsequent handshake may, indeed, have been a gentleman's agreement for him to win the WWE championship at some point.
I can't tell how serious WWE is about Sheamus. I'm somewhere between mildly serious and completely laughable.
It wasn't so much Sheamus was hand picked to win MITB as much as WWE has chosen to book Roman Reigns from the ground up.