@KurtGodel7 The coastal guns honestly sound like more more trouble than they're worth in order to get the balance right. Unless you think it'd be a really fun enhancement to the gameplay that you're determined to see through, it's probably safe to skip it and just give the invaders the round one penalty you have in mind.
Off the top of my head, the only major WWII warships I can recall being sunk by coastal forts were the cruisers Blücher and Königsberg during the invasion of Norway. Realistically, coastal defense batteries should be able to deter small raids, but not pose a serious threat on their own to a large invasion fleet.
But, suppose Britain builds a strong battleship fleet, and those battleships can exert more force than the coastal guns at a given hard point. Britain's response should be to unload one unit as an invasion. The goal would be to trigger a combat between the battleships and the coastal guns, with the British player knowing this would do more PU damage to Germany than to Britain.
If you want to avoid this kind of cheese, you might be able to turn the Coastal Battery into a building instead that doesn't get shot at in combat but does get captured if the province is taken. That would also be a natural way to put a one per zone limit on its placement. It could have AA style fire (1 die each at up to 3 ships, or however many targets you think is right).
If you think Claude can write some more complex rules, you could have the BBs take bombardment shots at the Coastal Battery first - if they score enough hits, they don't destroy it, but instead suppress it, and prevent it from firing. You could have the degree of suppression scaled to the number of hits it takes.
For instance, you've given the Coastal Battery shots at 4 targets by default. The British BBs arrive and bombard first - they score 2 hits, so now the Battery only takes 2 shots at enemy ships.
Making it a building also allows it to be a strategic bombing target, giving the enemy an alternative way to deal with it instead of just sending their ships in to slug it out head to head.
