Well, here's my take on these topics.

1. I think those caverns already exist in the earth. So when earthbenders stomp their foot to make a chunk of rock fly into the air, they're putting pressure into the earth which in turn pushes back up--the benders just control where and how large that portion is. Or when they cause the earth to slide and move, its just pushing on the other earth around it, taking advantages of spaces underground and compacting the miniscule compartments already there. Anyways, something so deeply scientific not only takes away from the 'legendary' feel of bending, but would be very hard to portray in a cartoon intended for older children that are still children nonetheless (although it has obviously appealed to much older ages ^^)

2. I think this is mentioned in several other bending threads, if you like reading through them. But it's been mentioned that Toph can only metal bend because she's an amazing master and does stuff that no one else can do to begin with. Kind of the same with Katara, and bloodbending is also limited only during the full moon. And as for like, bonebending, I guess it depends on your definition of 'earth.' Which minerals, how much of them, etc. Otherwise, minerals in the body are so trace...I personally find it unlikely.

3. Yes! I think so. Definately possible. Thing is, that was back at the beginning of the world when bending first came into existance. It's more of an origin myth. Unfortunately, you don't see it happening anymore. Spirit intervention has dramatically decreased...though I personally had hopes of air bending returning through such a process...

4. I don't have any idea about this. I wouldn't think so, though. Gravity is so much more, and what would gravity bending even look like? How could you do that? *ponders*...

Anyways, just my take of things. Feel free to contradict