K., We had the same problem with our son with all the same results (alarms, etc...they didn't work. ) and the same heartaches & frustrations (couldn't have friends over or spend the night anywhere, etc) (They didn't have 'pull-ups' back then! After all the tests, etc., were said & done, the doctor concluded that he simply slept so hard that the 'brain waves' that are supposed to trigger you to wake up couldn't 'get through.'
He put him on some medication that actually kept him from sleeping so hard.
He also indicated that we keep an eye on what else was going on in his life.... school classes and/or events, home situations with parents/siblings, etc., that might be causing some stress. We found very little correlation there. He DID seem to have more trouble when his allergies and asthma were acting up .... in the spring and summer.
But what worked best for us really was just limiting his liquid intake after 8 PM and making sure he went before going to bed and, of course, not making a big deal about it - NEVER EVER talked about it in front of his sister or friends/ family. Of course, if we were up in the middle of the night, we'd wake him (which was almost impossible - he, too, was comatose!)but .... he finally DID outgrow it.
We never knew what caused it. Our daughter (11 months younger) didn't have that problem. I regret all the tests I put him through .... it was really traumatic for him. I feel like we should have just tried to find a way to deal with it between us.
Good luck, K.!