Can't talk for your DM, bor, but here's how I'D do it if I ran your game.
First, she's a demon now. You're gonna need a wish/miracle to reverse that, and it ain't happening while she's bound. This is a high-standard/low-epic deal, so don't plan on being able to DO it until 17+, but before then is a good time to build to it.
You're gonna need the cooperation of one of those resurrected gods of yours, possibly "bought" with a good arduous quest. Then, you go down to Hell, find big Mister Demon Lord, MURDER HIM BEYOND DEAD, as in, use holy artifact sword, given by Healing-Goddess for that quest you did her, to nail his head to a fucking altar. Then subdue your marilith lover, drag her kicking and screaming back to the land of the living, and drop a 5,000 XP, level-losing wishacle to bring her back.
How's that sound?