hi bochos,
you're on the right track. good way to get several thruths right & wrong !
alert height has nothing to do with a kind of mode that would arm and engaged, freezing some devices ... like the land inhibit for some failures, or ILS lock below 700 ft.
it's rather a decison point (sama concept as V1 for take off) that helps the pilot in a CAT III b having a failure at very low height. if it is at or less than 100 ft height, he will disregard the failure until aircraft stopped on the runway since he knows the aircaft will be able to complete the autoland safely (except a second failure affecting automatisms just after (the first being as well in relation to autoland equipements. bad luck !) or the red autoland warning)
see you
cyrille