At least change the description for "Map (Marcel Jauri)" - if it shows five places then the description should not say that it shows three.
About an alternative island: one possibility would be Isla Mona. The beach/port location currently assigned is intended for the "Assassin" storyline and is a copy of the Barbados lighthouse location, with a small tower and a warehouse added. But you could define your own beach as a quest location, then in "StartStoryline.c" it should be possible to change Isla Mona's reload to point at your beach instead of the regular one. Or there's Petit Tabac, though if you use that, it could conflict with the "Silver Train" sidequest.
Or perhaps you could use
@Bartolomeu o Portugues' new beach on Camahogne/Grenada. That's intended for his "Family Story" FreePlay quest but could be used here, though the reload away from the beach would either need to be disabled or redirected to a new quest location - they lead to the ruins of a mansion which would not exist in the 16th century and which is definitely just for "Family Story".
@Bartolomeu o Portugues: any objection to using Linda Praia for this, on condition that the player is not allowed access to "Bartolomeu_Mansion"?
As a last resort, there's always Cozumel, though again you might want to play with reloads unless you want the player to find the temple which is the endpoint to "Tales of a Sea Hawk".
Is this ship supposed to attack the player, or does the player need to go looking for it? If the
Pastenague is the one doing the attacking then it's trivially easy. If you send a ship to a locator which does not exist, it will spawn next to the player's ship, so all you need to do is give it a nonsense locator.