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Welcome Chris!

1) I consider Banggood good for purchasing Q8, although I do read lately about false availability. Make sure not to pay more than $40
2) You can certainly get legit cells from Banggood. However, depending on where you are I would get lithium ion cells from a more reputable vendor.
3) There is low voltage protection in the Q8, so you can use unprotected cells. However, low voltage protection only works when the light is on, and not when it is in stand by. So if you’re not using the light for an extended period of time, let’s say longer than a couple of months, then make sure to remove the batteries or mechanically lockout the light by loosening the battery tube a bit; you open the electrical circuit so there is no parasitic drain.
4) Leaving the cells overnight in the charger should be safe, however it is highly advised not to do so. Never charge lithium ion cells unattended.

Cheers.