Qs on Low voltage driver

I'm planning to Replace the driver of either a Ultrafire C8 or Keygos KE5 with the one from a Solarforce 3mode(Hi,low,strobe) Low voltage dropin, I will also replace the Led with an XML. My goal in this is to be able to use a Single AA/14500 battery when needed using an adapter.

Some questions

- Is it bad for both the driver and batteries when using 18650 li-ion and draining them till the protection circuits kick in? (I know it affects the life of the battery but how bad)

- The driver have a loud sound when on low, should I be concerned?

- Seems to Direct drive on high, at what output should I expect from a XML T6 led?

The obvious problem I know is that when during usage and the batteries protection circuit kicks in the flashlight turns off, happened to me during a mt climb/hiking trip at camp, My L2 w/ diffuser served as our camp light then all the sudden it turns off as if there is a power outage, total darkness.