yes
yes but you will need to unlock it.
No, the iPhone 4 is not the only iPhone that will work with Verizon. All iPhone models, including the iPhone 3G, 3GS, 4, 4S, and 5, will work with Verizon.
yes!
Yes, they should be able to. The iPhone is backward compatible. Yes, they can. I used to have an iPhone 3GS and all the games work on my iPhone 4.
I have a 3G iphone with ios 4 and it didn't work on it
Yes. Both the iPhone 4 and iPhone 4S work on Verizon's network. However, you will need to buy the version of the 4 or 4S that is compatible with Verizon.
yes
I have an iPhone 4 and I listen to Pandora on it all the time! It works perfectly!
If you have wifi
any
Probably not, it could work vice versa, but a #GS is too thick to fit in most iPhone 4 cases.