Don't see an issue, but you already know that turning off the switch will cut power to both sockets.

2.5mm is oversized, so no issue to extend another socket.

Your new diagram is completely different from the original diagram. Are you sure the wires concealed within your walls are wired that way?

Using a double pole versus standard single pole in this case makes no functional difference. It only supports higher amperage - from 13A to 20A.