All the defenders were dead (AFAICT), and I used the minichopper to fly out. Apparently the 'trick'* is to kill everyone with sniper rifle | kill dog first | trigger alarm & take out choppers first. Since I saved with a looted RPG swapped for the sniper rifle, I'll try the chopper takedown method.
*I've since read that there is no 'trick', it really is a bug (for the unweakened fort), and your only recourse is to keep doing it over until it takes (or you takes it).
EDITED: 4 Jan 2019 20:38 by DSMITHHFX