Switch Won't Turn On (Unpatched V1)

I'll get the unpatched part out of the way first. The serial starts with XAJ7003 which I believe is unpatched hardware that can be more easily modded. I don't intend to do this but it may be that the previous owner tried and bricked the console.

I bought a used "junk" switch from Japan and it failed test #1 - it doesn't turn on. No charging/battery icon and no signs of life. I did a bunch of tests listed below, and any parts or tools used are verified working from my store bought and still working fine Switch.

1. Plug it in to charge for a few hours (official Switch charger to wall and USB to PC)
2. Use a fully charged battery from a working Switch
3. Hold the power button for 20s and then press again
4. Open up and clean all ribbon cables and re-apply thermal paste
5. Try to power on while docked in case the screen wasn't working
6. Press the power button directly on the board/ribbon
7. Test continuity on various things with multimeter (I don't really know what I'm doing here but everything I tested had continuity)

If I plug it into my computer it's listed in Device Manager as APX (!). Not sure if that means anything.

The internals were all very clean and beautiful other than a tiny bit of dust, which hopefully suggests nobody has opened it up to tinker before me.

I'm hoping someone could give me advice on what to try next.