Ah, now all becomes clear about the seat removal problem! What a shite design of rack! I think an AluRack and adaptor plate is the way to go or take a hacksaw to that offending bit of stupid plastic flange/lip/whatever it is!
Of course, that is a Givi Mono'lock' system rather than a Mono'key' system which mine is! I don't know if that makes all the difference. Just shows Givi never tried to take the seat off when they 'designed' it!