2am and can't sleep, wtf.
1) Preference. Personally, if I could marry the A+B button I would.
2) Sliding 236B? No clue what you're asking...though it sounds like you're wondering how they do the A-versions, in which case you're not adding 6 but rather 9 or 7 (or 8 for neutral). If that was the question, then go practice mode for a bit and be ready to superjump a lot till you get it.
3) Can't get the 2nd/3rd roll in? I'd guess you're hitting the 2C too late so the roll doesn't connect. What I do is hold 2 as she's falling, and try to hit C right before her feet touch the ground. The 2c is usually at the right hight then. Don't try to rush the 623 input afterwards because it easily (at least for me) turns into a 236B, which sucks.