I did some searching around, and it looks like this problem is more common than I thought (with this charger). I don't think I am able to determine the exact voltage of individual cells however, unless I'm missing something. I am going to try the NiMH charge again at a different charge rate and see if I can bring it back. Nothing to lose at this point.

As for the question about the ESC,
Default setting is 3.2v per cell, so for a 4s lipo the ESC will shut down when the total voltage drops to 12.8v, or if 3.4v per cell is used it will shut down at 13.6v etc etc.