@hnyman - outstanding! Thank you for building those images. I've got your factory and upgrade EA8500 files downloaded.
After reviewing how I did the initial EA8500 OpenWrt install, my plan is to coax the case open again and flash OEM version 1.1.3 (an older version that accepts OpenWrt factory) via USB to serial dongle and tftp.
Then I will flash the OpenWrt factory image that you graciously built from the old OEM firmware followed by sysupgrade and see how it goes. After confirming success and posting the before and after you requested, I'll leave the case open and your build on the device until the hopefully happy day I see your commit land in master.
With that, I'm going to leave you (and me) in suspense until tomorrow. It's getting late in my evening to start this tonight after a longer than expected day.
Thank you again for your efforts, plus moving this topic to its own thread.
EDIT:
It occurs to me that if OpenWrt can flash its own factory image, it would be superfluous to reinstall the OEM firmware first (I already have OpenWrt installed on the EA8500 in question, as would most users). Reflashing the OEM via tftp first seems safest to me, as that is how I originally installed OpenWrt factory, but if someone can comment on whether this is unnecessary, I could instead test a straight flash of the new OpenWrt factory with a larger kernel partition from the presently installed OpenWrt with the smaller 3MB kernel partition. I'd rather test and demonstrate a process that will be simpler for many other users than test a more complicated approach that re flashes OEM, if that is unnecessary. Comments welcome.