Here's where I'm at WRT the floppy.
First line 2010 should have the logic reversed force_read set is true. That changes error 12 - not read to 28 - rx50 board failure. That error is caused here.
The head load line is shown in the schematic here
but you can see on the next page it isn't pushed out to the drive. Page 3 seems to show that the motor for the selected drive is enabled implicitly by the head load line going high. The question I have though is what is happening with the Z80 though. I'd guess that the head load status bit on the 1793 isn't being set until the motor is up to speed (which would be reflected by the ready line if it weren't forced high on line 274 of the disassembly) which is possible as that bit is the HLD line anded with HLT, but the schematic doesn't seem to reflect that.