Just for curiosity’s sake, I applied a slitting saw to the oldest defunct generic NP-FS11 battery pack, cutting carefully along the bonded joint between the two parts.
No coolant, 1000 rpm, 200 mm/min, the saw is 22 mm diameter. Much slower than you’d use if you were in production, but I’m not.
First cut all the way around at 0.5 mm inside the case, then another pass at 1.0 mm. The second cut went ting as it passed the tabs at the base of the cells, so I knew the halves were released.
Inside we find a pair of 14430 Li-Ion cells, wired in parallel, with a little protection circuit board just jam-packed with teeny parts. One may reasonably assume the circuit controls over-charge and over-discharge, as well as current limiting.
So a reasonable (or, perhaps, amusing) thing to do would be to buy raw cells from a nominally reputable supplier, do a heart transplant, and see if that improves the situation.
Photos of the protection PCB, showing the cell connections. Positive end of the cells is toward the PCB. I think there’s enough clearance in the camera’s battery compartment to allow a wrap of tape around the case in lieu of re-bonding the plastic together.