Could get a little hairy. As part Treasure type, you can play it in the Buy phase. But what if you choose to play it as if it were an Action card? Surely that would be illegal, since you can't play Action cards during the Buy phase. But this says you can! But the wording is like BoM, in which case BoM itself isn't played, it's only what was chosen...
I don't think it would be playable as an action in the Buy phase, but it would cause confusion.
"until either leaves play" is odd. When could the other card leave play without Magic Mirror leaving play at the same time? Ummm... Urchin, I guess. And also if MM becomes a TR/KC/Procession and is used on a Duration, when the original was not. But then what happens to MM? Is the Duration still doubled? It's simpler to have MM be that card until MM leaves play, without consideration for the other card (which only matters in rare cases and is confusing then).
As far as power, this should probably cost a bit more, maybe $5. It offers a fair bit of flexibility and it usually never ends up dead, if you play at least one Treasure. Being able to emulate things like KC, Goons or Platinum is really good.
(Edit: looks like your edit addressed a few of these questions.)