If this dream island, which belongs to the current owner of the authority of the pathway of telepathy, is destroyed, the owner will die in the real world and the Mind Palace will drop out of them.
Once the Mind Palace loses its current owner, it will return to its previous ownerless state in the Mind Palace, so the god of dreams won't be able to use or control it even if this dream island is destroyed.
If the god of dreams wants to control the Mind Palace, he will have to find the body of the owner of the dream island so that he can be there with his angel of telepathy that he possessed to take the Mind Palace or hide it when the current owner dies.
The need to take control of the Mind Palace, if only temporarily, is why the god of dreams possessed the first angel of telepathy. Without this angel to do his bidding, killing the current owner of the Mind Palace will only return it to its previous state of being ownerless.
