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
Thanks. I have not been learning the vosotros verb forms, although I include them in the conjugation tables I've collected. Duo doesn't actually teach that form, either, probably because non-Spain Spanish is much more useful in the Western Hemisphere, so I'm still searching for an answer to the nos. question outside Spain.
The RAE doesn't know of nos. (but apparently nos is used for high-ranking people, see point 3), and the Wiktionary doesn't give any hints on such an abbreviation either, so I'd frankly say it's uncommon, or at least unofficial. But I can understand abbreviating it, nosotros is a pretty long word for a pronoun.
Thank you for the reply.
The places I've seen *nos." is in two very old books which supposedly teach basic conversational Spanish. I inherited them from my step-father. They were published in the 1920's. The use of "nos." seems to be a convenience for the books, but they are teaching books.
I have not seen this usage elsewhere, and my original question was posted a year ago, in September 2016. So, I think you're definitely right - it's not a real abbreviation in formal or even informal Spanish.
However, I'd bet that most native speakers would know what it was if I used it. So, if I were writing a postcard or were reporting something where I had to use nosotros a lot, I'd probably go ahead and use nos., and the heck with the grammarians.