Have I understood this correctly? You are asking if "Yes" is a valid sentence all by itself? And the same for "No"?
What an EXCELLENT question!
I would say yes. That's just an opinion, of course, not a rule I looked up somewhere, but I reckon I can justify it. Some languages - the Irish language, for example - do not have the words "Yes" or "No" in their vocabulary. The
Maybe these are special interjections in the same family as :
"Oh!" "Hurray!" "Curses!" "!"
They are special because they can take any kind of punctuation. They just happen to be rational interjections, not emotional ones (at least not usually...hehe).