1. Regulators were in the picture as an example. Was the only picture that I could find fast. No real need unless your voltage is really high.
2. In theory, the reciever has no way of knowing which channels have a battery on them and which channel to draw from first. Thus they would be drawn from at the same time and at the same rate.
3. Dual battery setups have been used for quite some time. I do not think that you need to worry as even digital servos do not draw enough to burn the reciever. It is possible, but not likely.

Hope this helps. Maybe others here can give more than my limited knowlege.

Jason