Joel, you are quite correct about the stray fringe field lines. I was being
somewhat sloppy in my writing. It's hard to say things in a way that is both
lucid and fully qualified against all possible contingencies and
complications. I was considering the infinite plate short plate separation
distance approximation. I also did not include any perturbing effects on the
charge distribution due to other possible charges and fields from other
nearby objects. For instance, I did not include the perturbative effect of
the field of the battery and of the induced surface charges on the wires that
connect it to plates A and D either. (I also did not include the fact that
charge is quantized and not infinitely divisible. Nor did I include leakage
effects between the capacitor plates either.)