Seriously, where is Regiorra? It's a central point of reference for all the Radical Dreamers locations. Is it another island, apart from Zenan?
I have a theory based entirely on information in the Encyclopedia here. It says Viper Manor was unknowingly constructed on Zeal ruins. To me, that means look at where the Ocean Palace is on the map and find the closest relative position. It's the continent on which the city of Medina resides, near the south, where Melchior lives. Regionna could be the name of the continent. After all, it's widely accepted that the Zenan continent is called Zenan and not Truce, the city. Maybe he just owns a little shack and that house you see on the CT world map is Viper Manor? Maybe that's a stretch. But think about it.
From: Theory (Locations)