Yes, that is a problem. Ash = denser than a hunk of lead. Though, he did seem to notice Melody's good looks in the second movie, and there was Giselle. *shrug* I think that for Ash to realize any 'feelings' he has for a girl, it would all depend on the girl. I think Ash realized (somewhat) that Macey had a thing for him at the Silver Conference, most probably because she was very obvious about it. For Ash to know that someone likes him, he's going to need a thwack over the head with obviousness. Just staring and blushing ain't gonna cut it. Ash seems more like an action person, so I think perhaps hugs/hand touching/kiss on the cheek would probably be effective.
As for what could happen to make May/Drew not canon... um, I think the most likely thing for that would be that Drew ends up training in one spot only, or leaves to a region that May isn't going to. It would be her decision though. She's just sort of following around Ash, so it would be interesting if she had to choose to follow Ash's path, or Drew's.