Look they changed the spelling in 1994 and no, I don't know why they thought switching between a and e was necessary
"Mirror, Mirror" is a more direct translation of the German "Spieglein, Spieglein" which is why it's more popular and ended up overshadowing the "magic mirror" line
No, it was never spelt that way. That spelling makes no sense. It's not even pronounced that way dead god why. (Goes for a lot of "but it was spelt [x]!" effects, but especially the "dilemna" one oh god that one hurts my eyes)
Sometimes people (subconsciously) switch out words or just mishear lines (especially in songs) if another word would make more sense. They expect that word so much that they just don't hear what's actually being said/sung. Especially in a/the switches where a definite article would seemingly better fit the context.
Memetic mutation of quotes that never appeared quite like that bc it gives you a fucking context. (People may not know you're imitating Vader if you don't address them as Luke)
Ok I don't know why people remember it that way but I'm guessing you're all conflating it with something similar? Like I bet there were a lot of figures with a top hat, a suit AND a monocle but the monopoly guy just wasn't QUITE one of them. So I guess the same applied to a lot of other cases.
1) you might be remembering a live version where he decided to sing it that way & 2) bc the chorus ends like that the previous times it's what you expect and it sounds incomplete without it so covers would tend to have the last chorus end that way too so you'd hear versions like that a lot