Back on the subject of Beauty and the Beast, it's interesting that when the Broadway Musical was created, they changed the prologue to say that the enchanted rose would bloom "for many years," completely removing the 21st birthday aspect of the whole thing. This is a quick fix that resolves most of the whole timeline mess, and one that I accept as canon just to maintain my sanity. :lol:
I truthfully just believe the timeline plot hole in the animated movie as an honest mistake that occured in a movie that was really being rushed to be completed. I think I remember reading that the animation studio just barely got B&tB completed by their deadline (you'll notice that they had to "borrow" animation from the end of Sleeping Beauty for the scene of Belle and the prince dancing at the end - they just did not have the time to create new animation). Considering the amazing movie we got regardless, I'll take an unrealistic timeline. Really, I think the whole thing could've worked and I could have easily believed the prince was 17 when he was cursed if it wasn't for Ashman's "10 years" lyric in Be Our Guest. I imagine it was just a case of no one really thinking about the lyric and the fact that it didn't really make sense as they worked to put the whole movie together.
Of course, this doesn't address any of the other plot inconsistencies you all have brought up, but I've always considered the timeline one to be the biggest problem, and the only one I couldn't shrug off with the "eh, it's a fairy tale, it works" excuse.