I used a cutter knife to start and then plastic scope and at the end near the contacts again the cutter knife. From my experience it is the best place to start at the back of the cover and go forward to the contacts.
I didn't found replacement cells in that size up to now. One cell is dead, one cell is swollen and the third looks good.
Thanks for posting the photos! I used similar tools to open a couple of Mantis battery packs. It would not have been possible to replace the cells and secure the casing for flight use.
I found the cells in about the same condition as you did. I think pushing the cells to 4.4V as the OEM Mantis charger does really shortens their useful lifespan. I tried charging to just 4.35V/cell at 0.7C and the packs are less prone to swelling, but are at about 90% charge according to the Yuneec Pilot app telemetry.