Doom never enters into an agreement without a plan to screw over his "partner". So here's how he puts one over on the demons: The demons never had a claim on Valeria's soul, they just wanted Doom to kill her. So Doom found a way to trap her soul. Once he's done with the demons, he can just clone up a new body for her. This kinda also explains the end, here, with the demons coming for Doom.
It seems in character for Doom, to try to pull a fast one on the lords of Hell. Also, to brutally kill someone he loves, sure that he can just bring her back. Maybe he'd erase the memory of her death, just to be kind. That's Doom's type of kindness.