Why do you doubt it? I mean its awfully reasonable . I think its his mother, personally, because the amulet that showed Harry the Ways back in Changes held her voice. Though its possible that its Lea, since she is Winter and instructed to take care of Harry(or his body).

I had a different place for Maggie Sr. to show up (by the way, thank you to all of you who post regularly, i haven't had this much fun dissecting a book since college).  When Harry materializes, the only thing he doesn't have is his pentagram amulet.  Maggie Sr. gave him the amulet, and then in Changes the insert which contained her knowledge of the Ways... so does it make sense that she's the "parasite" who's making his blood flow on Demonreach?  Perhaps a shade of herself (like Lash was) is actually imprinted on the amulet... which would be really fun if Harry were to give little Maggie his amulet to help her in her training when she discovers her magical powers...

Then again, i might just be suffering from sleep deprevation  ... thank you, thank you, thank you Mr. Butcher - these are amazing.