If you do decide to make the warlock use cha instead of con, consider 1 level of binder. The Neburius vestige allows the binder to heal 1 point of ability damage per ROUND!! allowing you to mostly negate the problems with the hellfire ability.
Indeed, this is used by many hellfire warlocks regardless of life status and is a prime example of how writers fail to check other splatbooks when releasing new mechanics.
