So I will respond as a console player to your questions, and I have a question or two of my own.
1. In YI2 grabbing that shell is luck in my experience. Do you know about the 5-up method? You don't get the shell to play with, but you get 5-up instead of 2. That's what I do, but if you can ever manage to consistently grab the shell I think that would be more impressive.
2. Play a whole lot. Then you can tell if you sped up or slowed down after you did a dive.
3. Someone else got this one? I never had it consistent.
4. Jumping off the shell is possible, but incredibly hard. The pswitch is probably out of the question for several reasons. You only have one frame to press the jump button when you hit the pswitch, you have to be pressing down on the d pad so you can duck under the fire ball. There isn't much time to do this because pressing down also grabs the fence. You have to catch back up with the pswitch after you drop it. Turning back for one frame then forward is the more feasible method for doing this, but I still think this won't ever happen in a console run. Feel free to prove me wrong.
5. I'm not totally sure what you are asking. If you mean when they kick it then swim up and grab it higher so they go up faster over that sleepy fish then you just press up and release the shell, re-press x/y and press b/a (both is nice) while still holding up. Maybe someone else explain it better. Side note: I had to look at my controller to know which buttons are which :P
Are you going to fly through the stairs in the ghost house? With lots (and lots) of practice you should be able to get it a good percentage of the time, like 1/4 or something. That would be Way Cool.
Are you going for the second cape to fly during the Bowser fight? It shouldn't be too hard if you don't actually fly up there but time it so you jump up real high at the right time and float back down. Finding a second cape that you can consistently get could be challenging.
In DP1 do you get the cape the way the TAS does? If you do this it would probably make up for not knowing if you are fast flying, and with practice it isn't too difficult. I think the jump to hit the cape-a-koopa is something like the 5th downbeat after entering the level.
EDIT: thegreginator the rest of my responses, including to your below post, will be at the SDA
topic.