So far the ondroid list looks like a dead end, the Insignia reader (NS-CR2021) that was conforming working does not support 1-bit mode, so while it mounts a drive letter and reads the chip capacity correct of course it does not read the data. I’ve ordered another one (Ugreen) for good measure, but don’t have much hope about it.
I asked Ignas and he believes the mmcblkNX does support 1-bit mode, so I might try that next (once I figure out how to load the mmcblkNX schematic in KiCad:). One problem might be that the mmcblkNX board runs both Vcc and VccQ at 3.3V which is fine for the functional eMMC I use for testing, but it might not with the potentially unstable one I’m trying to save.