In your place I would probably go ahead and go, especially since you seem to see your DH as part of the problem, and not the solution, so sending him in alone is not likely to be a constructive course of action.
I would be prepared to respond to that comment about the homework, because if your DS did the work competently in class, he clearly understands the material. I would let the facts speak for themselves, and keep my emotions out of it as much as possible.
And I would be prepared to push off anything that comes from the meeting that's new and controversial, with a "We need to see what comes out of the dysgraphia evaluation before we can leap to any other conclusions."
