OK I did some more research on this.
I tried flashing multiple times both from from the command line using sysupgrade -v /tmp/xxxxx
where xxxxx was the 23.05.2, 22.03.5 and 21.02.07 and from the luci interface which to my understanding does exactly the same thing. I even tried the December snapshot but that caused Luci to crash so I reverted back to 23.05.2
IN ALL CASES the OpenWRT image ended up on partition 1.
IN THIS thread a user swears that they tested flashing and were able to flash to the alternative partition on an EA7300 v2:
even when asked point blank if they were able to do it on an EA7500 v2. But another user in that thread CLEARLY has the same problem I do - which is that THEY were unable to flash to the alternative partition, either on a EA7500 v2
Now the thread above was talking about a Linksys 8100v2 and one of the users said that for that device, the partition layout is hardcoded in the .dts and the second firmware partition is not used.
So, tentatively, what I have arrived at is that the EA7500 v2 is_not the same as the EA7300v2 even though the model numbers are close. For the 7500, OpenWRT will NOT flash an upgrade to the alternative partition. Possibly, the Linksys factory firmware won't either - or the factory firmware WILL in which case you have a trap-door situation where once you get OpenWRT on partition #1, you cannot get it on partition #2, because OpenWRT's sysupgrade will only flash the partition it's booted from and the Linksys factory firmware will NEVER flash the partition it's booted from.
And in my router, the one time I tried flashing from the Linksys firmware on the Alternative partition, I got an error that it could not flash because the upgrade was already in progress. So somthing got scotched there.
And on top of that - IF my router happens to have a bad NAND block on partition 2 - since the OpenWRT firmware has a bug in it (listed above) where it can't flash to a bad block on this device, then I'm screwed even more.
As for getting any sort of error back from the command line, I'm screwed on that because the very first thing that sysupgrade does before it unmounts the flash is it kills the networking and sshd sessions. This is really, really shortsighted because there is no way to see what the heck is going on. POSSIBLY if I tore the router apart and plugged in a serial level converter to it - I might get some more information.
But, it's absolutely not worth doing for a router I only paid $6.99USD for and only plan on using as a wireless access point.
The only value all of this has been is to churn out a list of bugs for this router and a design failure decision for sysupgrade, and, possibly an informed detailed analysis of what is going on for any dev who wants to dive into it.
The archives seem to have multiple postings and threads from people with the same problem as me - one guy mentioned he bought 6 of these and they worked fine and the 7th threw fits just like mine - so I am hopeful this thread will help the next guy to come along with one of these routers.
And that next guy might just be me.