Mahan's "Decisive Battle" theories were essentially obsolete after Tsushima. The submarine was a factor to be sure but aircraft, the locomotive torpedo, mines and wireless all played a part.
I would venture to state that by the time Dreadnought slid down the ways in 1905, the battleship was already obsolescent but naval Mahanian dogma prevent most (but not all) admirals from facing the unpleasant facts. The swan song for the gun-armed capital ship was 8 December 1914 when Sturdee annihilated von Spee and, for the last time, established uncontested sea control using big guns alone.
As for the rest, I think your analysis is spot on. The Type XXI, XXIII and I-201 could never have been decisive for the reasons stated. Will go even further and say that likewise, Bismarck, Yamato, the I-400's and all their ilk were dodo birds trying to contest mastery with rats and dogs (metaphorically subs and aircraft); they represented resources, material and manpower wasted on theories that were irrelevant decades before they were built.
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