You're right, the RESET switch is inside the AV port where you can feel the click when press it with a toothpick. For me, I can't reach the RESET switch thru the tiny hole on the bottom with a toothpick.
Switch inside 3.5mm socket, one very good tool is cotton swab (cut head away)
Bottom reset switch can push with needle (thread hole side is better)
You said you would flash LibreElec to the NAND as your first step. That's what I've been trying to do all along, without luck. I've tried the various options, but none of them have worked. Can you expand just a bit on how you were able to get it to boot into the SD/USB to install LibreElec, without using the reset? this is impossible
You must do this reset procedure first time, even you start from sd-card. (if you update android version, you must press again first time)
Remove power cable, put in sd-card, keep reset switch on, plug power plug in, wait starting, release reset switch.
Some boxes not boot right from usb-stick. Flashing stopped in error.
If you want install in nand, you must start first from sd-card and then install it via Putty (copy installtointernal file and execute it).
But this is old information, i'm not sure if direct flashing with reset switch is now possible.
Try extract files in sd-scard.
don't forget copy device your tree file
It is bit difficult understand, first boot Librelec from SD-card and Flashing Android in Nand memory use same reset procedure, only files are diffrent.
Openelec boot in kodi and Android NAND flashing in Recovery menu, where you can flash it. Some Firmwares boot direct in NAND flashing.
I need more information, maybe it depend naming zip in Update or extracted files.
I'm pretty sure you must press reset swith, because i have s905, s905x, s912 and all not boot from sd-card without pressing. Later pressing not need.