Will iMessage and Facetime merge into a single app in iOS 7 and OS X Mavericks?

You will know like all of us when Apple release IOS7.

No. At least not in iOS 7. In fact, in iOS 7, Facetime gets its own separate app on the iPhone for the first time instead of just being integrated with the phone/contacts.

You can find all the information about iOS 7 that's available at http://www.apple.com/ios/ios7

I think not, and see no benefit.

How would you exchange iMessages with someone else while on a FaceTime call using a combined iMesssge/FaceTime app?

iMessage is combined with text messaging where it belongs.