If Antwerp has become the new supply base for Monty, is that permanent? i.e. does it stay as the supply base if one of the A towns prerequisite is lost, or lost and taken by the Germans?
Not exactly an answer, but keep in mind that Axis counter-attacks can't happen adjacent to an army supply base. That may not make it impossible for an "A" town to be lost after Antwerpen becomes the army supply base, but should make it a lot harder.