I also believe probably there is some short, and that explain why safety is triggered in batteries. one of the things I will not do in this point is to put unprotected batteries.
u have to find where the short is. Am not familiar how tk75 is build internally but first thing i have to check is battery compartment, if he can be pulled out do the short again happen between + and - ?
if yes problem is there, if no than continue to check the path the + goes till the driver and to the led. somewhere in this path must be a short circuit which tigers safety mechanism on batteries.
from what you say i think to things can happen when you put on bag. one is like you say the flash turn on but it heated up very much being modified and lacking fresh air to cold so led can go dead but i thing is very improbable that ALL three leds go dead in the same time, so probability driver have gone dead heating to much and probably is shorting something. or some wire unsolder-ed from heat and also make short somewhere.
or if u drooped it hard somewhere something in battery section have moved and is shorting from plus to ground, but this is probably less likely to happen.
two batteries at 3.94v seems to have worked some sec at high before something go puff and short inside…