Have you tried changing the slot? 
$ fastboot getvar current-slot
current-slot: a
$ fastboot --set-active=b
Since your bootloader is still unlocked, this should be fixable one way or another 
Have you tried changing the slot? 
$ fastboot getvar current-slot
current-slot: a
$ fastboot --set-active=b
Since your bootloader is still unlocked, this should be fixable one way or another 