Oddly, all can mean exactly the same depending on which word you stress. However, a simplistic breakdown would be.... "We are going to return" and "We shall return" mean pretty much the same: that you will return but there's an unsaid implication that you may not. "We will return" implies that there is no other outcome other than your return.
Nosotros and Nos are different words entirely.
Nosotros = we, Nos = us / to us.
There's some discussion about it here which might help: https://answers.yahoo.com/question/index?qid=20091004085408AA66aEg