@vince
Welcome!
Each cell contains several paralleled sub-cells in the same housing. Please lets use battery, as it is common practice, for an assembly of one or more cells.
From a BMS perspective, battery consists of 8 cells. How many sub-cells per cell is of no concern to BMS.
however with 1 dead cell the other 5 are actually getting overcharged by 17%
Absolutely not! Charge is voltage limited, and _not_ volume limited, one dead sub-cell out of six would just reduce cell capacity (and thereby usable battery capacity) by 1/6th.
One can't really comment on what to change, prior to understanding what caused the failures.
Yes, these cells are rated to -18C only.