You can almost always use the AP and if it doesn't work you'll know. Under reduced system ops, you'll be using the AP down to 500' only anyway.
Abnormal Law is another name for Alternate Law.
Basically you get: NORMAL, ALTERNATE, DIRECT (LAW).
Within ALTERNATE law there are some sub-laws with reduced protections (low/high speed stabilities) and some without protections.
You'll also always get a reversion to DIRECT law when L/G is selected down (except with Abnormal Pitch Law).