The "BIOS:Disk 0 is NULL" debug message is telling you to try mounting a floppy image as the A: drive. Might not work if there is copy protection extant, but by now you've seen enough cases where it does work. Yes, there is a flood of additional messages, but you can capture the messages into a log file.

Had to try this myself, and indeed it runs fine from a floppy image mounted as A:. Curiously, if you *boot* an MS-DOS 3.30 floppy image with the game on it, it fails with the same symptoms mentioned above.

Yep, the game tries to write an IRET instruction at the INT 13h handler address, then calls INT 13h to read a sector with the carry flag set in advance, so an error will appear to be returned if the INT 13h handler is in RAM instead of in ROM. Fun fact: DOS version 2.x and earlier leave INT 13h pointing to the BIOS handler in ROM, but later versions point it to a DOS handler in RAM that chains to the BIOS handler in ROM. So, I think what the game is doing is anti-deprotect rather than anti-DOS, particularly because it uses DOS FCB functions, but the mechanism only works correctly on the oldest versions of DOS (and in DOSBox, because it has the handler in ROM).