Posse Comitatus makes no sense at all. It cannot possibly be constitutional. What the hell is the military for if not to protect the homeland?
One thing I know... PC was a congressional measure and what congress passed congress can jolly well repeal.
Posse Comitatus was penned and approved by a much more literate Congress that was raised on "The Gallic Wars" and "Julius Caesar". They feared a military led by the charismatic generals of the day marching on Washington, DC and having a coup.
It made perfect sense then and it still does.
BUT, there is no reason why the National Guard could not be used to accomplish these things.