I have the same receiver (2310) so I should be able to provide some input.

1. HDMI control is NOT what turns the other components connected to the receiver on or off (and vice versa). HDMI control keeps the last HDMI input you were using powered when the receiver is in standy mode. This allows someoone to watch TV using the TV speakers if they do not want to crank up the receiver and surround, for example. There is a seperate setting once HDMI control is turned on which is responsible for powering the receiver when another connected component is turned on. I don't remember the name but it's under HDMI settings in the menu, and I know it can only be used when the HDMI control is on.

2. Pretty sure this is Audyssey Dynamic Volume at work. I noticed this as well but have never paid much attention to it.

3. Leave the Denon in "auto" and displaying Dolby Digital. In the speaker setup menu you should only have it set to the speakers you have. If there is no center, the receiver should automatically direct center information to the front L & F.