Thank you for the reply Stuart, but no luck.
I did download Mr. Miller’s image. It would not boot at all until I replaced the files that you mention, but the kernel in that image locks up after detecting the fourth core of the CPU. However, from that failure I learned that those files, (start_cd.elf, start4cd.elf, fixup_cd.dat, fixup4cd.dat) are necessary for the Pi to boot, and that those with the bootcode.bin and presumably, but it doesn’t seem to matter whether I use bcm2711-rpi-4-b.dtb or bcm2711-rpi-400.dtb - the dtbs are vital to the process. - and that all those files simply need to be copied into the fat partition/boot directory.
So I burned another image (actually many, trying different SD cards, and different configurations, older kernels, etc) and replaced all the files I’ve mentioned with the ones from
https://github.com/raspberrypi/firmware/tree/master/boot (hopefully that’s where I should get them). My most recent iteration just has the single error repeated:
sdhc: read error intr 2008002 stat 1fff0000
This occurs many times. In the middle of these errors is
/dev/sdM0: BCM SD Host Controller 02 Version 10
then the error repeats itself over 50 times before printing out the lines
/dev/sdM0/data
bootargs is (tcp, tls, il, local!device)[]
At no time during this process is the keyboard or mouse responsive. Though the mouse icon did become visible during the boot process.
I am hoping I am wrong, but I am thinking there is some sort of driver issue. At the very least, checking what media there is to mount, or reading the SD card. And then possibly for other things, but the former could be gumming up the works for everything else.