Not likely- even in ALttP, the thing is touted as being functionally omnipotent, so if sacrificing seven maidens descended from the original jailers could get him out, a wish should do so nicely.
2. He got Agahnim into the Light World the same way he got his malice into the Light World during the Seal War
Either during or after the seal was cast that closed off the connection between the worlds? At which point Aghanim settled in and twiddled his thumbs for a couple centuries, before moving against the king in earnest? With no one noticing him in the intervening years?
3. The Sages probably created the Magic Mirror at some point after the Seal War
I'm fond of a similar idea, that being that the Mirror (and possibly the Moon Pearl) were actually involved in casting the original seal. But that's headcanon- the game itself doesn't offer that explanation. The mirror is just kinda there, conveniently.
Black Velvet Inferno