I have to agree with The Cool Thatguy. I don't think Ren being the big bad would have worked. Especially not have the last movie went out of it's way to empathize with him.
I don't know about that. I mean if they chose to be nobody they probably chose to change thier last name as well. Granted that's speculation on my part. However I don't think the name change in universe was meant to be anything but a screw you to the Emperor. Out of universe it was of course to make the title fit.