But, if Descartes were right, and there actually was a fundamental, causally potent mental substance that drove human action by exerting force on the physical brain, and all ordinary matter besides the mental substance obeyed the physics we know

Does it matter if you use 'mental substance' to get around the issue of how the brain does things, if you then do not know how the substance does things? Computation is computation, no matter the substrate. Psychology rejected the concept of the homunculus for good reason.

There is no action without reaction. What properties of the brain would be necessary for it, and only it out of all the sorts of things in the universe, to be able to act upon the mental substance?

Of course, these questions are not germane to the issue at hand, because your 'mental substance' is casually active. Chalmers' consciousness isn't.

Who among us has not yet accepted that Chalmers' argument is fatally incoherent? Let them step forward and speak.