A lot of questions

7. Check out the Media Texture channel
8. Check out the Go FullScreen channel
13. This is possible, you can make a smart system for it or hardcode it (both in quest) but it's not that hard to do (use a trigger -> depending on the selected button, set one value to 1 and the others to 0). Regarding your original question (multiple cameras), you can achieve this by using a Channel Switch rather than a Channel Caller. A caller will call all attached channels; a switch calls the channel specified by the first child (being a value ranging from 0 to infinity).
14. I believe Act-3D prefers fx composer - but they both work.
Please also check out the Tutorial Manual for some of the basics, I think it will get you pretty far to where you want to be.
Happy channeling

-Derk