Hi J.,
My son, who is almost five now, did the same thing when he was 3. He would be dry all night but did not want to stop during the day to use the bathroom, and like your son, had many accidents.
You are right about it partly being a power struggle. It's the one thing these little guys do control versus everything else in the world that mommy, daddy, or other grown-ups control. So, in that case, the more we push, the more they push back and just do their own thing.
Also, it is a maturity thing, and we cannot force maturity---only help guide it along, and this takes time. You are on the right track, keeping a sense of humor about it, doing common sense things like having him use the potty before you go out and about, etc.
At this point, going back to diapers could send mixed messages to your son. The only time I put a pull-up on my son after he was trained was one night when he had several episodes of diarrhea, and I did not want to be changing sheets all night. I just explained it to him that since he had been having diarrhea, we were doing this just in case he could not make it to the bathroom in time and that this would not be something we would continue to do. That was the one exception.
One thing that might really appeal to your son as it did with my son is the Elmo potty game on the Sesame Street website. I learned about it from my son's preschool teacher. I'm not really a big fan of having my son on the computer, but I was willing to try anything that seemed positive and had the potential to help. I'ts a cute little game that has several versions where Elmo and his friends are playing and one of them has to use the potty. Elmo says, "When you feel the feeling, stop what you're doing and go, or you could have an accident!" Then, the character will go and afterwards, comes right back to what he/she was doing previously. My son still quotes this when he has to stop playing and use the bathroom. When' he's done, he will say something like, "Now, I can go right back to doing what I was doing before! It had that much of an impact on him that he is still refering to it almost 2 years later. He still occasionally asks to play the game.
Maybe if it comes from Elmo, it will help break the power struggle thing. Your son will be hearing the message you're trying to send, only it's not directly coming from you this time.
Just a thought and hope it helps. I wish you luck and patience through all of this!
J. F.