I believe the battery failed due to a combination of both. I have overdischarged batteries during runtime tests before and the discharging is not the problem. It is the charging that has to be done carefully and properly, in an appropriate charger. The maximum heat did not kill the battery on the spot either, as it survived and started cooling off, before it failed. So I guess the combination of overdischarging the battery while it was so hot, killed it. The battery registers 0V and will not take any charge even when directly connected with a full battery, so it is completely destroyed.
Thank you very much for your offer of a battery for the cause. The battery shipping problem remains, so I will keep looking locally, but your kindness is heart-warming, as always.
I disagree with you on your conclusion. I believe the temperature thresholds implemented in this driver are brilliant:
The temperature threshold for pain due to heat is about 46C. This means your tissue will have to reach 46C for you to start feeling pain. The body of the light barely reached that temperature, without anything conducting heat from it. If you hold the light, it will never reach 46C, as some heat will flow into your hand, cooling it. If you leave it to reach 46C and pick it up, you will experience the slightest discomfort before it starts cooling off.
The ring will indeed reach 56C if the light is left to tail stand long enough, which is definitely uncomfortable to hold, but then you pick up the light, it will start cooling off gradually, as heat will flow from the body to your hand and from the ring to the body. It is safe to adjust the dial by touching the ring briefly, as the temperature differential of 56C to your body temperature is not high enough to cause such high flow of heat that will raise your tissue temperature to uncomfortable levels, from a brief contact.
So, unless you do it on purpose, leave the light to tail stand until it reaches full temperature, then pick it up and immediately and continuously hold the ring, there will be no problem.
On the contrary, this is the exact thermal regulation I would use to make the light thermally safe to use, without sacrificing too much of its performance, which is already not the highest. In other words, the thermal regulation implemented allows the light to operate at the performance limit the host can sustain, while at the thermal limit the human tissue is comfortable with (if used normally / wisely).
If they had implemented overdischarging protection in the driver as well, it would be nice! As it is, I will purchase more 2020 drivers.