My repo and key setup is correct. Librewolf has somehow messed up their repo setup.
I removed arm64 from the Architecures section, but apt search is still showing only the arm64 version. I decided to ignore this and checked whether I could install librewolf via synaptic - nope, the arm64 version shown by synaptic is really the wrong architecture. Then I tried apt install librewolf - which worked. With the correct amd64 version. Synaptic now shows both versions, one unnamed version installed and the arm84 version as uninstalled.
Thanks @sunrat for their help that enabled me to circumvent the upstream repo mess. I should really report this to upstream but that would require me to create another account somewhere ... if anybody else stumbles over this error they'll find the solution here.

Create an SD card (or USB drive if you plan to use USB Boot) in MBR mode with a single partition of type 0x0c (FAT32 LBA) or 0x0e (FAT16 LBA). Then format this partition to FAT32.

Note: Do not try to use GPT for the partition scheme or 0xef (EFI System Partition) for the type, as these are unsupported by the CPU-embedded bootloader.

This explains why I was unsuccessful in converting Trixie to Excalibur by eliminating systemd (the process was challenging as well).

I do not hate Wayland. I have no personal beef with it. I don't really care. I'm not a developer. I am not bothered by software. It's a means to an end.

What I don't like is ANY, I repeat ANY software solution that champions mediocrity.

For example, Skype 8 over Skype 7. Microsoft removed the sane desktop-oriented Skype 7, which let you do multiple chat windows at the same time, and replaced it with a pseudo-touch "modern" turdling that could not do this thing, and many other things. New solution, inferior to old solution. Missing functionality. The end user suffers. Simple.

5. Remember, this is a meeting place every that shares a love of Devuan, and Linux (without systemd).  So please play nice.