1. It's based on HD.
2. Yes, you can.
3. So far as I can tell, the only limit from the feats is Constitution score -10.
4. Constitution, almost assuredly.
5a. Yes, it can
5b. RAW, yes, unless they specifically refer to the class soulmelds (Incarnate will boost the capacity of any soulmeld, but only Totemist soulmelds can be bound to the Totem chakra, for example)
5c. Yes and no. It does not count for the class limit, but it does count for the "Con-10" limit.