1. Because AT&T (and Verizon) in it's arrogance, has decided to implement it's own virtual wallet, which, I think, is only being trialed in a few places.
2. Not sure, sorry. I'm not on AT&T
3. The standard Email app should support both. I use it with my work exchange account without any problems. I'm not currently using it with an IMAP account, but it should support it.
There is an app called Touchdown that integrates with Exchange Server somewhat better than the stock app, but it's pricey. They have a free trial though, so if the stock app doesn't do what you need give it a try.
4. Read these forums, and the blogs on Android Central.
The Getting started thread:
http://forums.androidcentral.com/gen...ps-tricks.html might be a good place to start.
The Help and How-To articles might be a good place to look next:
Help, How-To, and Tips for Android | Android Central