So the US forts are all magically worthless, the veterans experienced in littoral combat choose to stay home, and the thousands of men with a great deal of experience in mine warfare (with the recent invention of the Hertz horn making them far more lethal) make no use whatsoever of their talents?
Yes, the Third System Forts are hopelessly weak against modern warships, a fact impressed upon US planners when the British smashed the defences of Alexandria to rubble in a day, and Alexandria was much more strongly defended than any US port.
The "infernal machine" is hardly new, and the US are hardly at the head of the curve. In fact I'd be at a loss to remember whether the Union ever deployed any, the Confederacy certainly did, and their effect was hardly spectacular.
Because of course there shall be no mobilization prior to or during a war crisis
Oh, give the US six months and they'll have 200,000 men under arms, which is enough to man the coastal defences...
Given six months lead in the British can still mobilise a force of well equipped regulars to meet any US mobilised militia with their civil war era rifles, guns &c. We forget that not much later the British sent 12 divisions (what the US would call an Army Corps) for the defence of a colony. Put another way, the British of 1899 put more troops into South Africa than the US was able to muster in a full mobilisation in 1898.