Thanks. Replacement battery has arrived, but I haven't installed it yet -- hoping to understand what's going on before possibly ruining another one.

With
the old (swollen but sort of working) battery connected at J6950
USB power connected (through the one good port)
the ribbon cable that goes into J6951 disconnected

the voltages at pins 2 & 3 of J6951 (on the board) are both 3.4V.

As before, it negotiates 20V goes through what we called phase 1 above, and in phase 2 now power cycles between 0 and 250mA -- I assume trying to charge...