recently Robogodzilla was featured with other Mechagodzilla's at the Godzilla Interception Operation Awaji Museum so does this possibly mean its simply just another name for mechagodzilla?
It's not too terribly different than something like Salunga. Or Cyber Godzilla. A monster can take inspiration from another without literally being the same character.
But, Robogodzilla appearing in a Mechagodzilla display is a bit more convincing. A compromise could be to continue to call him Robogodzilla but put him in MG's portal.