Continuing my attempts using my slimmed-down example (
https://github.com/mmguero/deblive_blank) I am now getting the BIOS installer to use my preseed correctly, and get through the "copying data to disk" phase. Now debian-installer keeps exiting and being restarted. The error I see is:
main-menu: WARNING **: Internal error! Cannot find "debian-installer/main-menu doesn't exist" in menu