Q4 - The alert height is typically 100 feet above minimums no matter what the approach type, it has nothing to external or internal system integrity. There are also no guarantees that a systems will never fail, so you can always go around. But the reality is, weather limitations mean that if you have a system failure after selecting reverse you are at a point where the aircraft is rapidly losing speed and the aircraft is quite controllable. After all, you will still have 75 M RVR.
Is alert height not suppose to be only for CAT III 3B with NO DH and it's 100 feet above touch down on a 320 and 200 feet above touch down on a 330, thus reference to radio altimeter. Don't know about other types.