Haven’t read the entire thread, but have you checked the spring in the tailcap? You’ll need tweezers to remove the inside of the tailcap.
Edit - o.k, now I have read the thread properly, so it’s not the tailcap spring.
I had a very similar problem with my ultrafire C8, it went dim and checking it on a multi-meter there was very little current. I ended up replacing the driver thinking it was a driver issue, however it was actually the LED that had gone bad.
If I understand it right, the J18 has a single positive and negative wire from the driver connected to the LEDs, the LEDs are wired together and complete a circuit/loop. If one of those LEDs go bad, then the circuit won’t complete or barely complete, causing the entire flashlight to go dim.
Problem is I don’t know an easy way to determine this. Looking at my LED and starboard it gave no clues that it was bad, solder points were good, however it was sitting loose in the pill so had poor thermal transfer.
One thing that should be easy to check is to remove the reflector and check the LED and LED starboards are firmly seated and don’t move around, if one is sitting loose then that could be the problem. If it is an LED problem and they all look fine, then perhaps someone with better knowledge may be able to suggest how to isolate the problem LED.
You could try turning it on and carefully checking each emitter to see if any seem to be performing differently, presuming you don’t blind your self, however considering the entire circuit would be effected it may be impossible to tell.
Of course it may just be a driver issue, however considering the odds, 1 driver vs 7 emitters……………