I must say I have enjoyed this thread. R'lyeh being one of my favorite nations to play since Dom2, I have still learned a few tricks from this one
I have tried out Teleporting in Starspawns with great success. 8-10 of them, with 3 forming a communion, 2 raising undeads and 3 simply chaincasting soul slay works great to punch through lightly defended provinces. 1 turn later I will have gated in enough forces from my capital to defend it against anything but a major attack with mages and SCs from a good human opponent.
This does not work against a fortress though, since the Starspawns can't breach the walls by themselves. But in multiplayer I can see the advantage of having 2-3 groups of these raiders as a mobile reserve to bolster the defences where you 'think' the enemy will strike next.