1. I used some Atlantic speaker stands, the type that have storage for CD/DVD's, but I had to modify them to allow me to screw-mount the speakers to the top plate. Even after all that, they are top heavy and would fall over easily. I should mention, though, that perhaps the biggest improvement in sound quality came when I switched the LF and RF speakers to stands rather than having them sit on the shelf on the equipment stand, so by all means I suggest you find some speaker stands that will work. If you have kids or pets that might jostle the speakers, I would suggest something other than the Atlantics I used.
2. I mounted my center speaker to the top of my TV with some foam rubber mounting pads that have sticky tape on both sides. An alternative that I wish I had thought of before doing that would be to go to a hardware store and get some velcro with sticky tape on the backs. That way I could remove the center channel speaker without destroying the mounting system.
3. You will need to program your receiver to operate in 5.1 rather than 7.1, and you will lose the sometimes noticable additional sound from the rear surround channels. Not much movie/music out there is encoded in 6.1, which the 790 will process into 7.1, but when it is available I think it makes a huge difference compared to 5.1.
One thing you might think of if you don't use the rear surround speakers for surround purposes is to use them to take advantage of the "Zone B" option. You could use the surround speakers in a different room to create a stereo system in there; just be aware that they will have little bass output. For background music, though, they might just be alright!
Good luck. I've been following your posts and it sounds like you have a great start on getting the system installed. Let us know how you like it when you do.