10'X12' Mirror Glued To Wall
No idea how to actually remove the mirror, as there is no telling what kinda glue was used, and no way to know what kind of glue coverage is holding the mirror up. I would suspect it is held up with liquid nails or something similar, and most likely they used up a full tube, shooting a wavy line till the tube was empty.
Having said that, I would buy some cheap contact paper and cover the whole mirror first to prevent the glass shards from flying when the mirror breaks. And it will break...
Maybe you could score it with a glass cutter and get behind it with a putty knife, if you're lucky you could control the breakage. Some heat on the putty knife may help too.
As for whats behind the mirror, expect the worst; if you don't have to cut out the 10 x 12 drywall and replace it you'll be lucky, but you never know.
Good luck, and post your results.