I don't know where you get the idea that "nobody knows" about Paul's opinion on the ineffectiveness of the doctrine. I don't know any Paul supporters who DON'T know about it, and yet he's poised to win Iowa.
I found it interesting that you said that "if people knew he thought states should be able to ban guns or have whatever gun control laws they want", considering that many states basically do that anyway with the incorporation doctrine already in place. It's ineffective, so he enjoys the virtue of being right.
They’ve only just now incorporated it, give it a chance. It already stopped Illinois from banning handguns.
Believe me, most of Paul’s supporters have no idea he believes that. I know, I talk to them on reddit on r/libertarian, most of them have no idea what Im talking about.