I think the way they did it was fine. I usually prefer for them to constrain title switches (especially when it concerns the world title) to happen only on PPV, but I don't mind when they happen on Raw or Smackdown just as long as it's done sporadically. After a string of horrible shows and record-low ratings, they needed to do something out of the ordinary to get people excited and get them talking, and I think they've succeeded so far (especially with the legitimacy of Reigns' title victory and job security still up in the air.)
As for Wrestlemania, Reigns vs Triple H is the match with all the story and build behind it, so why anyone is seriously contemplating any other opponent for Roman is beyond me. As for how to book things at the Rumble, there's a couple of different ways they can go with this one:
1. Reigns is stripped of the title and the belt is once again declared vacant, and for the first time in 24 years, we get a Rumble for the vacant WWE Championship. Don't see them going down this route, especially since it would revolve around Reigns capturing his second vacant championship within the span of two months.
2. Reigns keeps the title, defends it against Sheamus at the Rumble, and then HHH, having not made a single appearance on the show since then, enters himself into the match at #30 and wins it. Or if he really wanted to be a bastard, he waits for the match to end and for someone else to be declared the winner (and you can choose whatever babyface you want for this position... Ambrose would probably fit best for a couple of obvious reasons), and then Stephanie comes out and says this year has a special 31st entrant, and out he comes. He tosses out the winner, screwing them over and winning the match and then getting on the mic and saying that he's gonna take care of Roman himself at WM.