Part I: Is this just an ad populum? How do you account for sociopaths, psychopaths, or masochists? If I like getting into fights, should I assume you will like it too? Is it's popularity the sole reason it is valid?
No, I don't believe that its popularity means that it is valid. After all, belief in the necessity of government is nearly universal, but can be demolished by demonstrating its internal inconsistency. The Golden Rule has no such internal inconsistency in that it could be followed by everyone without conflict.
Part II: I will relate to you a story from last Fall--election night actually lol. I was out having dinner at a little restaurant, with my girlfriend and my family. As we sat, enjoying some chips and tea, there was a screeching of tires, as a young man was struck by a car. I left the table, and ran into the middle of the street to offer assistance, and found the teenager laying, in obvious pain, and definitely suffering a concussion. A nurse who happened to be driving by also came to help. The man was so injured as to not know his own name, or where he was, or what had happened. He wanted to stand, but if he had a spinal injury, that could be fatal. Now, according to the NAP, I should have let him stand up, and just walk it off. It became quite apparent that was a bad idea, and instead, I chose to violate the NAP, and lay on the ground, putting the guy in a gentle embrace, and carefully restrained him. He cried and begged for me to let him stand--and fortunately he was very weak from the accident, or I would have had to let him go, for fear of causing direct injury. Paramedics came, and they carried him off. I later visited him in the hospital, and he thanked me for restraining him against his will.
I obeyed the Golden Rule, and forsook the NAP, did I not? Was it wrong to hold him against his will?
No, you did not forsake the NAP, as you were not acting aggressively, i.e., attempting to harm him or take something from him for your benefit. Instead, you did your best to preserve his bodily integrity, and succeeded. That is the farthest thing from a NAP violation I can imagine.