Who speaks for the IMG? Who speaks for GMG? Neither are tied into a house very strongly. GMG are linked to Kusari slightly, but retain independence.
The GMG fought an 80-Year War and won against a mighty, militant power. I think they would strongly dispute any claims that they were not a nation... and I doubt the Rheinland government would admit losing to anything but a sovereign nation. Therefore, GMG would be considered a nation, for the intent of this Court.
Now, if the GMG are a nation, then the IMG would say 'what about us?', and would receive Kusari and Corsair backing on that front. The IMG would come up in discussion so often that they'd need to be included, especially considering Tau-31, and their allowing the Nagasaki to park itself outside Holman.
The Mollies, now, would want to be seen as a nation of their own, the nation of Dublin. The Bretonians would dispute this, but would allow them to come to the table for talks, as they may have to side with the Mollies, giving them concessions to get aid against the KNF. They both, with different agendas, would need different representatives.
EDIT: Please get Juan Arquero in here! Ace67's corrections had errors and he made his own mistakes too!