EGPFlyer,
with regards to the loss of an RVR reading, for CAT 2 and CAT 3A Ops EASA just says that if the runway has 2 or more assessment units then 1 can be INOP. So by that the TD RVR could be INOP, honestly can’t find a reference to whether the midpoint can/can’t be used instead (as long as it’s above the original TD minimum RVR).