1. I can't say why white supremacists gravitated toward his campaign. Possibly because his Libertarian view that you can believe what you want as long as you don't hurt anybody else appealed to them. As for those "Israel-haters," I can probably say with a little more certainty that they were drawn in by Dr. Paul's view that America's love affair with Israel needs to end. He says to give them the same treatment that you would give anyone else around the world. Meaning no preferential treatment.
2. It's hard for me to comment on this one, because I simply disagree with you. I've never personally found Dr. Paul's knowledge of the Constitution to be lacking in any way. Feel free to elaborate.
3. I can't understand how so many people confuse isolationism with non-interventionism. Dr. Paul believes, as Thomas Jefferson believed, "Trade with all, entangling alliances with none." He's willing to be diplomatic with nations such as Iran, Russia, and China. Diplomacy and commerce are not products of isolationism. You'll have to be more specific as to what you saw in his ideas as isolationism. I would also say you're over-simplifying his foreign policy by a large degree.
As for being a footnote, well, only time will tell. I will say that many are beginning to see him as somewhat of an economic authority in the past few weeks, so if nothing else that may be his legacy.