IW, you are trying to make sense of Hudlin's terrible writing.
This was a mess of a story that was confusing. IIRC, there was arguments and the usual Namor bashing and blaming at the time, and this later issue by Mayberry tried to clarify it / rescue the mess. It's clear from the panels in the first issue, that T'challa did NOT leave Namor's alleged island (which doesn't look like an island or in his taste). He barely got out of the doorway before Doom ambushed him.
I don't think Doom gifted Namor an island. First off, Namor doesn't need anyone to give him an island. There's plenty of islands in the ocean he probably claims. The island in this issue where Shuri ambushes him, appears to be one. Plus, we know it's not an island near Latveria, because Latveria is landlocked.
I really don't think Namor and his army was in Latveria at this time -- though I think Hudlin thought he was. Remember, Namor wasn't in Latveria, but actually in jail for stupid reasons, which happened in Fraction's The Order. But he was freed, when the Cabal was formed, and Doom was free and back in Latveria, Namor and Emma made their deal in Uncanny X-Men Annual -- and all that happened before this Black Panther issue. It is at times like this I really miss Comicbook Database.
Given Namor's farewell to T'challa, I often wonder if Hudlin actually DID plan / write Namor as betraying T'challa to Doom, because he wasn't quite in synch with the rest of the MU, and especially with the X-Office, which had already claimed Namor. Plus he was leaving Marvel at that time, and Mayberry finished this volume of BP with Doomwar.