I believe the evidence at this point agrees with you: NAND failure. Pretty strange given that both boxes were working fine on the previous release, with no hint of trouble until I attempted the upgrade, but... must be a difference in how the memory is used by the image.
I tried a number of different combinations on both boxes. For the most part I was able to toothpick to 7.95.3 and then upgrade to 20190226, only running into trouble with 20190424, but then I started to see similar problems even with 20190226.
I installed to SDCARD and am running 20190424 without issue.
False alarm, sorry to cast aspersions on your release. Really appreciate your work.