I think the Fleet SA needs a tweak. If it is only 1 mine prelaid then it should be a given that they can. Not a 50/50 chance of it happening, I have seen mines have a minimal real effect apart from psychological
In games where I have used fortifications the opposition ignore them. How about a fleet SA like this - installations count as fleet for destruction purposes. All installations may offer an extra ranged by 1 square AA or Main once per game.