@Match, I wouldn't go that far. ;) But here my responses are. (In italic as usual)
1. When having 2 or more 18650’s in a light from what I understand is that they should have close to the same voltage, my question is; could you use 2 batteries with same voltage but different mah? Ex. 3100mah 4.1v with 2600mah 4.1v
First, you have to be sure the light can handle them. In general you want identical batteries with identical use, this means that they should all be 3100 mah, have 3 cycles, and be 4.15 volts. This is most important in series, where the batteries are stacked on top of each other. In parallel, this is less important but my recommendation is to keep the batteries identical. Also keep in mind that the ratings on XXXXfire batteries are not accurate and should not be used in multibattery lights.
2. If a dropin says 3-9v on the side that’s just what the emmiter can handle and not necessarily what the driver is able to?
That is the voltage the driver can handle. Most emmiters can only handle around 3 volts, but the drivers will reduce it to drive the LED appropiately. Do not go above the listed voltage as this will most likely burn out the driver.
3. For a p60 dropin, if one didn’t want to take out the reflector again would there be any benefit or disadvantage of filling it in with copper, or another heatsink material, and sealing it into the head. Basically just the top of the reflector would be sealed into the head, regular heatsink for pill etc.
The best thing to do would be wrap the reflector in copper sheeting, but aluminum sheeting would also work. Or you could also use aluminum foil, whether or not you want it to be permanent it is a good idea to do this.
4. How do I know if the springs are too long to fit certain batteries? Just the resistance or will there be a crunch? Can it affect the battery? I’ve read some springs need to be cut.
For most lights it will be fine, generally if it fits it will work. If you have to try really hard to tighten the tailcap then it might be a bit to long but that rarely happens. It can put a dent in the battery but this will be fine, just do not try to "pry" it back up.
5. With 18650 batteries how long can you leave them in flashlights?
18650s have a very low self discharge. My general rule is leave them at 4.2 volts if I will use them at least once a month, if it is longer than that it is best to store them around 3.8 volts but either way it will be fine.
6. Can very cold temperatures affect the led? Ex. Wires shrinking
Depends how cold we're talking. :evil: Any reasonable temperatures found in inhabited parts of the world will be fine for the LED. If you were to place the LED in liquid nitrogen I doubt it would survive but I do not know. Match, you wanna test this one?