My answers aren't certain as I haven't researched or tested anything. But I don't think Flame Bodies stack, and I doubt walking into walls counts as steps (the pedometer in DPP didn't go up if you walked into walls).
As of Generation Four, it is now spelt Ho-Oh, instead of Ho-oh.
Always talking to the characters in White Forest does not make more things appear. It just prevents the existing ones from vanishing.
As the Regi puzzle of this generation, it can be confusing. When you first enter, get to the middle pillar in as few steps as possible. Talk to it, and it should move for you (it is recommended that you use the left entrance). On the second floor, use Strength right when you're near the pillar, and go up. On the third floor, use Flash. And then you're done! The key here is to take as the smallest amount of steps possible before the torrent of water kicks you out. If you need a map, use this one by Mew Jadester.
Edit: In response to below: Yeah, oops. Use Flash on the second floor, and Strength on the third. Thanks for the correction.
I actually used the south entrance myself, and it worked just fine. I know you need Flash and Strength, but the one for Flash is on the second floor, not the third.
But yeah, just finished getting everything there tonight.