And, again, I am not talking about the power source itself. I am talking about the ability to shape and direct the power regardless of where it comes from.
If it is as you say, then yeah, it's impossible for Maggie to pull off a death curse. There's every reason to believe that "setting aside" your magic removes your ability to shape and redirect energy, regardless of source.
My question is whether or not setting magic aside removes your ability to shape, but rather your ability to
sense. Less like a surgeon losing his hands, and more like his hands are shot up with Novocaine. It would make day to day magic so difficult that it's totally impractical to perform, but leaves open the possibility for doing something clumsy, out of desperation.
There is absolutely no evidence for this in the books, as the concept is only introduced three times (all three of which are related to the Carpenters, funny enough; Michael mentions it to Harry as a way to get rid of Lash, Charity actually did it, and Harry presents it as an option to Molly down the line, when she has a handle on her power. I'm sure there's an argument to be made that connects it to the Parable of the Talents, but that's outside this discussion, and not really relevant).
Anyway, I don't believe that Margaret's death curse was clumsy; it stuck to Raith, and magic slides off him. It was deliberate, focused, and effective in a way that other wizards weren't able to match since Etruria. Whether this is an artifact of its nature as a death curse or a testament to Margaret's skill I can't say, though I'd argue that it's reasonably likely that other wizards have fired death curses Papa Raith's way in the past. Maybe because it targeted Raith's connection to his Hunger rather than Raith himself? Or maybe because it was anchored to a bloodline, a persistent connection? Regardless, it wasn't the work of someone with numb hands; it was a sniper shot Kincaid would be proud of.
Though I do think the connection between Margaret's soulgaze message and Lash is very interesting. I haven't considered that in the past, but it really does seem like there is something similar about them. It wasn't just a recording—the pendant's ruby does that—it was a piece of Margaret. No idea how she was able to pull that off without understanding the way the Coins work, or at least the way something else we haven't seen does it.