[st.patrik]

I forget, can your Sanguine Heritage Vampires fly? If so that might be a way to get to the mainland.

As for defense, I think that's pretty easy, at least against R'lyeh: lots of PD to absorb attacks + squads of ranged troops (probably Rangers) set on 'fire archers' to kill Illithids. The bigger problem will be R'lyeh dominion making your commanders mad. LA Atlantis is more of a land nation, so they may or may not be such a threat (though they do have sailing).