Could you provide the reference that states latching of ALT2 permanently disables the autopilot?
AMM:
The AP can be engaged in EFCS normal or alternate control law, not in direct law.
If ALT is caused by double ADR FAIL the AP is not available (
see PJ2 #356) but after UAS the AP can be available.
One of the conditions to engage AP is CAS > VLS, For AF447 this means that after 02:10:46 (2 consistent and good airspeeds available) but CAS < VLS the AP could not have been reengaged.
In the 'air caraibes memo' AP2 could be reengaged after UAS while in ALT2x, although VLS was not displayed here, it must have been calculated by FMGEC and CAS>VLS.