2. Is there a "Reverse BASIC" XL OS ROM floating around than can be used in the U1MB like the one that can be found in the AtariMax 32-in-1? Asking for a friend.
Not really required with the current firmware, since if you enable the PBI BIOS (regardless of whether you have the HDD or high-speed SIO driver turned on), you can set 'BASIC default' to 'Disabled' on the 'Memory and System ROMs' menu.
3. Relatedly, can BASIC simply be switched off on the U1MB or can a slot be left empty so that there is no built-in BASIC at all? Or do I need to set my machine to OSb in order to do this?
See above. You could also probably make a BASIC ROM which simply sets a few OS variables and then disables itself by writing to PORTB via a few bytes of code copied to RAM.
4. Does anyone have a NTSC compliant OSa ROM that has been padded out to 16K that they could share?
OS-A-NTSC-16K.zip 7.92KB 5 downloads