There are two blown resistors (one of them shown in the picture) that correspond to the non-detected cells. These are 0.01R resistors on the input of the balance leads, before the mosfets. I think these are used here as a fuse, so it did it's job
Yes, its' failure mode is to short circuit the gate to the drain, so it's prudent to have some sort of 'fuse' here. Most car audio amps have the exact same protection.
Interested to hear your outcome with this, most likely a failed mosfet. I have unit on the bench to repair as well.