First of all, I think you are fully qualified to use Li-ions. Remember to respect them, but don't be too afraid.
Especially after scaru's incident, I'm not confident with any light not accidentally turning on.
Luckily the S5 has anodized threads and it can be locked out by opening the tailcap about 1/3 turn. That's what I'm always doing with mine when putting it to backpack (or similar).
FWIW, I use unprotected flat top NCR18650B in my S5. Fits well & offers good capacity.
Over voltage protection is provided by the charger and under voltage protection by S5 driver. Possible dangerous event would be the light shorting during use, but then the user can notice & act on that. (If the light turns off for any reason, open the tail cap and remove cell.)