OP - I totally sympathize. My son sounds very similar to yours and I am not the mom I want to be either. For 3 years I have grilled my son daily about "did you poop" and "did you pee" and "let me see that poop" and I can't believe the things I have said and done

So I totally get it, I really really do.
First, we had a terrible experience with one of the urologists at Children's. Truly awful. Dr. Kalloo is the head of their wetting and soiling clinic so she may be more helpful (assuming you haven't seen her already). Mary Micker is a nurse who also works in the wetting and soiling clinic and I have heard she is good too. We have not seen either of them so I can't say first hand, but it may be worth a try.
Second, you may want to try a bladder relaxing medication, you will need someone to prescribe it, of course. We had my son on Detrol for a while and it did help with the wetting, although one of the side effects is constipation so it sort of defeated the purpose. But if you can maintain a clean colon while on Detrol or Ditropan, you may have good success with it.
Third, we use a vibrating watch to remind my son to go to the bathroom every 2 - 3 hours. It works when he doesn't ignore it! It may help just get your son in the habit of going at certain times of the day. Ask your son's teachers to remind him to go at school.
Fourth, if you suspect any food allergies or sensitivities, I would suggest getting tested and/or trying an elimination diet. The things that tend to constipate and cause GI problems which can then lead to wetting problems are dairy and gluten. It also can't hurt to throw a good probiotic in his diet too.
I know you said you feel sure that the constipation is gone, but we have also been aggressively tackling my son's constipation for a very long time and he has always been very backed up whenever he has been x-rayed. Personally we are done with Miralax, all it did was give him very loose poop and did not keep him cleaned out. I think we are better off having more formed stools and giving the rectal muscles something to push against. We use suppositories if it's been more than a day since the last poop to make sure he's emptying regularly.
Best of luck to you. I think we need a support group!