Looking back at the original question about "I'm just saying", it was about the origin, not the meanings. Douglas seems to have a handle on the possible meanings, but not on the origin. Several people have explained what they and their friends mean, but not the origin. I envision some Brooklyn punk (or perhaps it was Ben in New England) making an offensive statement and pissing off some tough guy, who says, "Oh yea? I ought to kick your ass." To which the punk (or Ben) says "I'm just say'n", as he backs off while holding up both hands. It diffuses the situation and he learns to use it at the end of any insult he makes. It may have evolved from the statement, "I'm just asking." That also comes from some punk who ask girls if they will go to bed with them and when they get insulted, he tries to laugh it off with, "I'm just asking."
I don't know the origin either, I'm just say'n.