I think I found the problem, folks.
Using anything connected with WoWs as a source for real-life stuff is... unwise. Unwise on a par with using Wikipedia as your sole source when doing research. Wargaming is more interested in gameplay than accuracy, to begin with (hence their nerfing and buffing various ships for "balance" reasons), but beyond that, they want to fill out "tech trees" (for want of a better term) by making sure that all the nations have the same number of roughly-equally-capable ships (and, in WoT, armored vehicles) in each tier, with the one exception being that they deliberately unbalance it a little to favor whatever flavor-of-the-month they're featuring--and also to favor stuff you have to pay actual real-life money for (witness Massachusetts, a buy-with-real-money premium ship, having advantages over Alabama, which you can buy with in-game-only currency... despite the two being literally identical in real life).
This means that, not only do they frequently nerf and/or buff ships' performance relative to each other (hence the Montanas being essentially useless against the Yamatos in-game), a lot of the ships in the tech tree are napkinwaffen, not real ships. They admit that some of them were "never built," but what they like to claim is that they have super secret documents that prove that their designs were considered, at some point, but never built, for reasons. A fine example is the "Phoenix-class" cruiser, the US Tier IV cruiser in the game. They claim that it's a preliminary design that "never materialized, but informed the design of the Omahas". Friedman's exhaustive history of US cruiser design lists nothing that matches those characteristics; it actually seems to be something in between Design 112 and Design 116 of the April 1915 scout cruiser design studies, albeit with double the horsepower. (Essentially, 112 was an unarmored scout cruiser with ten six-inch guns; 116 was the same ship, but enlarged to allow a four-inch belt and a 1.5-inch deck, to protect against destroyer guns.)
Meanwhile, a fine example of a pure napkinwaffe would be the Japanese "Zao-class" cruiser, which doesn't appear to be based on any actual IJN design studies and is instead created out of whole cloth simply so that the IJN cruiser tree can have an equivalent to the Des Moines class; likewise, the "Hakuryu-class" CV is an entirely fictional design extrapolated from Taiho to give the IJN carrier tree an equivalent to the Midway class. While the Amagi-class design isn't fictional, it was overtaken by the Washington Treaty, with the two ships scheduled for conversion to aircraft carriers during construction, like the Lexington class was for the US; when Amagi was overturned by the Great Kanto Earthquake, she was scrapped and the similarly-Treaty-stopped battleship Kaga was converted in her place. (The Kii-class design is based on preliminary design studies only, having never gotten to the point of the kind of detailed design needed to actually build a ship.) Great whacks of the Soviet and German trees never got past the "back of the napkin sketch" phase, along with the Tier VIII and Tier IX British battleships (and the Conqueror seems to be pure fantasy). (Another example of WG creating stuff out of whole cloth would be the entire Chinese tank destroyer line in WoT; historically, China never had any TDs.)
So don't take anything from WoWs too seriously--and even if it crops up on their forums, take it with a grain of salt the size of your fist, because it's entirely plausible that any "design information" that someone from WG mentions on there is really just them either testing the waters for a new fantasy design they thought of, or trying to build up anticipation for a new variant they're getting ready to release.
The 18"/48 wasn't something that was being planned for; it was entirely a BuOrd project to develop a gun that would let them be ready should anyone else continue the caliber race that was ongoing in the teens, so that if the General Board demanded something more than the 16"/50, they'd be able to provide it in a timely manner. Likewise, the 18"/47 variant that they tested in the 40s was, again, not something seriously considered a realistic prospect, given the performance of the 16" superheavy shells; it was simply BuOrd going back and reviving the project as a fallback position should some unforeseen contingency require even bigger guns.
The only time any study of a gun bigger than 18" was undertaken by BuOrd, it was in connection with the Tillman designs; the request for designs included specifying 24" guns, which was then changed to 20" guns at BuOrd's protests that they could not provide any sort of accurate characteristics for a gun with such a large bore, at least not through extrapolation from known data. Thus, the Tillman designs were outfitted with 16", 18", and 20" variants, with balanced protection; the hypothetical 20" gun's characteristics and performance were extrapolated from known 16" gun data and estimated 18" gun numbers based on their well-advanced 18"/48 design studies. Of course, the Tillman designs were never meant as serious designs for production; they were a simple exploration of the limits of what we could build if Congress gave the Navy a blank check, meant primarily to shut up people who were complaining that the new battleship designs were too big and too expensive to keep building them when they were then obsolete a couple of years later. (Said people were saying, "Just go all the way to the logical end, build the ultimate battleship, and leave us some money for more useful purposes!"; the idea was to show them just how horribly expensive it would be to build said "ultimate battleship" instead of continuing to go with gradual escalation of the designs that leave them with obsolete-but-still-useful ships after each new iteration arrives.)
While there may have been brief consideration of putting the 18"/47 on a couple of the Montanas, it was never a realistic prospect; likely the only major difference between the Montana class design and what would have actually been built would be incorporating Admiral King's proposal for a more-efficient secondary battery layout to increase AA firepower. (Said proposal was under consideration for Illinois and Kentucky, as well; it basically took a cue from US cruiser design practice and removed the midships pair of five-inch mounts from the sides, instead planting them on the centerline, fore and aft, superfiring the main battery--the benefit was that this would not only increase the secondary broadside by two guns, it would do the same for end-on fire, as the midships mounts were screened from firing directly forward or aft by the presence of the rest of the secondary battery.) While I'm not certain it would have happened, it seems like a relatively minor change that could be made easily enough. Otherwise, the only things that would be changed would be the inevitable addition of approximately ninety kajillion Bofors and Oerlikon mounts, as happened with just about all US warships of the era.