I've heard a lot of different stories about Northern Calloway's life and death. It's difficult to know what the real story is and we probably never will. One thing I've noticed is that celebrities from Sesame Street do not seem to be subject to the same tabloid probing that other famous people are made to endure. It's just as well too because, while I'm curious about Northern Calloway, I firmly believe that everyone is entitled to their privacy.
"Follow That Bird" was produced before Northern Calloway died and was still on the show, so I am not certain as to why he was not in the movie. I really wish he would have made the cut because that is the only feature film that will ever do Sesame Street justice (IMO).
I don't think there was ever any episode that showed David and Maria breaking up. One day, Maria and Luis just fell in love, and David did not seem to mind at all. He and all of the other humans watched approvingly as they kissed inside the Fix-It-Shop. David did make a couple of references to he and Maria still beings friends and that it will be strange to think of her as someone's wife.
David's absence was explained by saying that he went to live on his Grandmother's farm. In fairness to CTW, Northern quit the show before he died, so the episode could have been produced before they had a chance to decide how to handle his death. Of course, David's memory did seem to be pretty much ignored. In the musical skit "Birdland", scenes with David sitting in the audience were edited out and of course, no skits featuring David are shown any longer. There was an episode in the 90's where Gordon and Susan played a video tape of Miles' adoption and David was actually shown for a split-second which really surprised me.
David was my favorite character and I sometimes feel that losing him (around the same time that Jim Henson died) was more than the traditional format could stand and that's why all the crazy changes happened.