From a design perspective, 1945 is a good end point. The cessation of hostilities and the influx of war prizes created a launching pad for new work. Perhaps the clearest example is in dreadnoughts; Stalin retained his ambitions, but the Kronshtadt and SoSo classes were completely cut off, despite the existence of some potentially useful hulls.

I remember once trying to find the rationale behind the nickname Robin, but I can’t recall if I had any success. In any case, we wouldn’t want to confuse her with the actual USS Robin, which was not quite as impressive as an aircraft carrier.

American carrier forces would have begun raiding targets in the Mandates. I don’t believe there were any facilities west of Pearl that could have supported the Pacific Fleet, and since Kimmel was not the sort of fellow to throw the battle fleet ahead of its logistics, there may have been some time before any decisive fleet encounter. The Philippines would have been an even tougher nut to crack, so the resources gained in the East Indies would have been more difficult to get to the Japanese home islands.
There’s no doubt the Americans would have intervened.

Thanks for looking that up for me. A difference of 100 lbs is a much more conventional figure, as I’ve found only a single reference implying that 20 lbs might be more accurate.
The British and Japanese guns are indeed the most powerful ship-mounted guns during the dreadnought period. To the best of my knowledge, the greatest range achieved with the 18in gun was 40,500 yards using supercharge propellants and increased gun elevation. The 46cm gun could reach to about 45,960 yards.
During the time when pre-dreadnoughts were gestating, there was an enthusiasm for so-called “monster guns.” You’ll find guns of up to 17.7in or so in various navies. These guns used shells that were light by later standards, and the muzzle velocities tended to be very low. Nevertheless, as a group, the monster guns were a flop with numerous barrel failures. The technology of barrel design was too primitive for the pressures involved, and the black powder propellant was little more than an explosive that threw the shell forward in one sudden heave.

What naval engagement has ever lasted for hours? Most are very short in duration.

Uh…how about Samar? Gambier Bay went to General Quarters at 0645 and capsized, still under fire, at 0907.

The battleship gunnery phase of Surigao Strait lasted maybe twenty minutes. Kurita’s ships would have been in battle for hours prior to reaching Homonhon Island. His ammo loadout was split among three types of shells. I’m not sure he would have had any advantage in AP rounds before engaging Oldendorf. I’m also not sure that using HE against Yamato was a bad idea.

On the Z Plan, I couldn’t agree more. In fact, I’ve probably slathered this forum with my angst on the subject on several occasions.
I guess it’s no shocker that when a Young Turk in the German Navy in WWI wrote an article bashing his own service and its visionless policy, the navy tapped another up-and-comer to write the rebuttal–a fellow named Raeder.

they weren’t even heavy cruiser class in terms of protection

I’m going to disagree with you, though you make an interesting point. Protection on the Deutschlands was pretty good by heavy cruiser standards. The first thing to notice is the thick armor for the main battery. I’m trying to think of ships that did better. Hipper (one of the only advantages to the later, larger ship) and Zara, of course. The big American ships from Brooklyn on. I think that’s all. No British, French, or Japanese ship could match it.
Deutschland’s belt and deck armor enclosed a lot of hull volume. I don’t know any Allied competitor with the same percentage of protected buoyancy. The 50-80mm belt is nothing to crow about, but it is inclined and backed by a substantial bulkhead. (I would have shaved 20mm off the bulkhead and added it to the belt, but nobody asked me.) The deck protection is twice as thick as Hipper’s. There is no perfect armor scheme, but Deutschland looks respectable to me.
Now, about the comment on follow-up ships. This is especially interesting, I think, because it leads into the shadowy chasm of German naval thought.
The next ship after Graf Spee, Ship D, was going to be an enlarged version of the same concept–six 28cm and eight 15cm guns, but faster and more thickly armored. The fact that it would have had turbine engines is the first indication that something was amiss. When France announced the Dunkerque specifications, Ship D underwent a complete redesign and emerged as Scharnhorst, one of the worst battleship designs of the dreadnought era. (Sorry there, lukeylad!)
Dunkerque has an importance in battleship design that is often overlooked. The last battleship class to be built before Dunkerques was designed was Nelson, that speedster with 23 knots. All the Deutschlands could hit 28 knots on a good day. So the underlying concept of the armored ship was not unrealistic. Dunkerque effectively raised the bar on battleship speed standards. The RN abandoned its “fleet submarine” program because of this, and even retired the “battle cruiser” rating from further use; the King George V class ceased to be called battlecruisers and were officially rated as battleships despite their high speed.
The Germans built no more armored ships because they concentrated their efforts on battleships, and they signed onto the international treaty system, which did not accommodate super-cruisers. There was one more plan to build armored ships, but like most of the Z Plan, it never reached completion.