Also i do still have the original bios in a matchbox i can try it if you think it might be unibios, i just need to take off the case (MV-2FS boards have a metal case)
I don't think that UNIBIOS is an issue anymore, although the Green screen with warning will not appear anymore. Try changing the BIOS for an OG one and let us know if it helps.
Also if i set the multicart menu settings to boot to games i get a unibios crash screen, if i soft reboot on the crash screen then the games work. However, then i can't go back to the multicart menu -
As explained in the instructions, if you set it to boot to games, it won't enter the menu. You need to keep 1P pressed all the time after reseting the Neogeo so that you will go into the menu again.