What will happen if last thief didn't die but the one died on a trap, then you restart. Which one will it disable, the trap you died on or the default trap. I've tried this and it was the default trap that was DT'ed.
It will be the default trap, because only the last thief's death is relevant. In case you restart, the DT potion will behave the same as when you restarted without using gang, ergo, it will remain the default trap.
The amount of health lost will be that of the trap the last thief hits (or the damage you take from restarting, respectively) too.