I made it !
I had to correct a couple of blunders (see the diagram below) and now the system woks (almost) as expected !
Not all the forbidden/impossible codes are prevented :
- IN and OUT seem to allow the IMM4 and REG modes, though the output binary code will be decoded "correctly"
- INV allows SND to be output
- probably some other obscure combinations are possible
However this panel is only one half of the ASM equation : the disassembler display will prevail over the buttons configuration.
This is why some markings are missing : when programming, look at the output and trust the display, don't stare at the buttons.
(I'll have to shoot a video...)