A: This could be for a various number of reasons. The most common is that your certificate was not set up properly in Keychain Access, or that you're not on 10.7 (Lion) or higher.
A: In order to succesfully codesign and deploy the application onto your device.
A: Sometimes. Most applications require the rootfs or libraries/frameworks (e.g. MobileSubstrate) that do not work on a non-JB device.
A: No, unfortunately it is not. This is strictly limited to the Mac OS.