a) Flexible conduit in boxes. We are going the other way, we had one range without the conduit, but now comes complete, our are 12mm.
b) One way over replacement of probe is a low level service box, 36mm metal back box with blank plate over. Extra work and extra item on the wall, but a half way point between the Controller and the end of the flexible conduit tube. You would then feed the probe from here to the end of the conduit, fish the wire to back of controller above/the other way. Make sure smooth radius to flexible conduit from wall to floor.
c) Repairs can be done, we have kits. The problem needs to be found first. If it happens while cable being installed repair can be done before compound goes on. Best way to guard against problems is for you to use a monitor / alarm. If a cable is nicked the alarm goes off.
d) Always have a good plan of your area and where the mats are going. The mesh does not stop any heat so does not matter which way round it goes. We like to tell installers to use wooden or plastic trowels, Not metal. As with any cable, for lighting, power etc, look after it while you are installing.