A few days after my first post and I have a couple of breakthroughs to report!
Thanks to a post I stumbled on elsewhere I found that my board's reset microswitch is flakey...and that by shorting its two pins on the back of the board I am able to activate the reset mode properly. This explains some of the inconsistencies I was seeing when trying to use the toothpick method to get the usb port detected, boot sd cards or go into recovery. So my device wasn't completely bricked and I got the usb burning tool back in business and flashed a working Android image to bring my MXIII back to life.
Opening the box also yielded better information about what hardware I have...the sticker on the board says:
NETXEON MXIII V3.1 S802/AP6181/1G/8G
I made yet another pass at reading this topic and I noticed post #1572 (thanks Asxetos) which mentions starting by flashing and old OpenElec image from recovery and then upgrading to a more recent Libreelec image using a tar file. After hunting down a compatible image (OpenElec-Amlogic-MX3.arm-5.0.6.zip from MEGA), I tried it and sure enough, an alternative linux installation succeeded for the first time. Then I upgraded to LibreElec also without any issues (LibreELEC-S8X2.arm-8.2.3-MXIII-1G.tar from the link in post #1 of this topic).
So my MXIII is now running LE 8.2.3 with Kodi 17.6, everything seems to work and, as I had hoped, it boots and runs *way* faster than my Raspberry Pi.
This solution won't help anyone not prepared to sacrifice dual-boot...sadly I never found a device tree (dtb) file or any way to make a LibreElec install image boot from sdcard on my hardware.
Now the next step would be to get an upgrade to a 9.x version of LibreElec to work (have not tried that) but as far as I understand the hardware platform I have is not being actively worked on by Demetris anymore (thanks alot for the 8.2.3 image though).
Sorry, but how come MXIII is not supported, when it says so in the first post?
I will release LE9.x images for only for the two devices i own - MXIII-PLUS/MXIII-G/PROBOX2-EX-PLUS - S82/S82B/S82H/MXIII/S89/S89H and that's it.