is there a sense in which the reversers do not just contribute to retardation but also assist the working of the brakes in themselves, such that when removing reversers from the equation the brakes, particularly with a less than perfect runway surface, contribute less even than the contribution they were making previously to the joint braking effort?
Not really. See it as the T/R add themselves to the drag of the airplane by contributing with a force opposed to the trajectory.
What aids braking by putting most of the aircraft weight on the wheels is the spoilers.