It's probability based. Higher values indicate more robust systems. Theoretically a fail op system could have no alert height but it would require a go around all the way down to the touchdown point.
As for system freezing, I'm not an AB guy but you don't want the pilots thinking about anything other than the runway in the later stages so system downgrades at low altitudes are just a distraction.
You will probably find that below the "freeze height" the list of failures that will give a NO AUTOLAND indication increases.
There is no point reverting to "APPR 1" or the AB equivalent. Better just go-around.
FlightDetent, are your aircraft Fail passive or fail operational?