Pics rotated & more or less aligned, makes tracking down a spot from one side to where it goes on the other side.
(click for full size 1364x678)
I offer this without any suggestion that it is something you should try, only that it's something I have used many times when tracking down a dead short in an unknown location, when it's not possible to find it in other ways. It's a last resort kind of thing, but, most likely it will only break something that's already broken.
I was a professional auto tech for over 20 years and the only thing more difficult to find than a dead short where the fuse blows instantly every time is when the fuse only blows seemingly at random once a month. A true all-the-time dead short can be found rather quickly, but you have to have some balls to do it, especially when it's an expensive thing that doesn't belong to you. But, it's already broken, right? Whatever it is causing the short already is in need of repair, but you can't repair it if you can't find out where it is.
This is one of an assortment of 'not-fuses' I built over the years.
(do you see where this is going...? :) )
I never burned up anything that wasn't already damaged. The action starts closest to where the original short is, and if you keep the connection going in short bursts, only just long enough to spot where the first wisps of smoke come from, it's fine. You can then go to where the trouble is, and fix it. Fixing it is the goal. What happens along the way isn't all that important.
Good news is, since this is a dead short on the input side of the driver, you don't need to use the full original 8.4 volts. A single unprotected cell will do just fine. Connect it in short bursts, around 1/4 second (or less) every few seconds, until the problem spot makes itself clear. DO NOT simply attach a battery and let it sit there while you go fix a sandwich. If you can, mount or clamp the driver in such a way that both sides are visible, and have a helper watch one side while you watch the other. Wear safety glasses or use a shield - if you do it for short bursts nothing should explode, but there are no guarantees.
In no way should this affect any of the proprietary non-replaceable parts (like the MCU with the Fenix-specific firmware on it), all the stuff on the input side of the circuit should be fairly common commodity components that can be replaced. But you first need to find out which component needs replacing, if you're careful it should only damage the part that's already failed.