I don't think Demonreach has a mantle-stripping quality.I know it's a fan-favorite, as a way to "rescue" Molly, or etc. Demonreach has a crapton of power, but entirely lacks the finesse and the understanding of Humanity that would be needed to separate a Mantle.
Again, we don't know. My feeling is, no, the mantle would not leave Mab unless she somehow can shake it, which I don't think is possible (unless very specific circumstances happen, at least)
I don't think it has to have, I think the mantle on it's own accord would go to the nearest available vessel.
Doubtful.Demonreach is all about capturing Powers. If Harry were to genuinely target Mab, her Mantle would be captured (stripped of her Mantle, Mab is likely a broken mortal shell).