for trial 6, there are two things that I can think of that you can try. You could try really, really hard to do a perfect DP motion (f, d, df) and then do the EX fireball (which will work, but itsn't always easy). The problem with doing DP motions is that a lot of the time, even if you are trying not to, you're going to be inputting them as f, qcf. If that is the case, then obviously if you do qcf+P right after it, you're going to drive cancel into his (qcf)x2+P super.
The other thing you can try is inputting the DP as hcb, f+C. I think that is one of the input shortcuts that the game recognizes. I know it is something that you need to do a lot in K`s trials, and I don't see any reason why it shouldn't work for Iori, too.
So to get dp+C [DC] qcf+AC, you would input hcb, f+C, qcf+AC, and you should get what you want.
As far as hitting the weird mid-air rekka after whiffing the first one in trial 5 goes, i think you're just going to have to keep trying it until it works.