The fact that I-401 is carrying around a Wave Motion Gun normally reserved for at least heavy cruisers (hers came from a battleship, even) became less of a surprise if you dig around a bit in The Other Wiki. The I-400 class of Japanese subs are the largest submarines of the WWII, and continues to be the largest subs ever built until the Soviets build their nuclear-powered behemoths in the 1960s. They are capable of circumnavigating the globe one and a half times on a full tank of fuel, and actually closer in concept to a submersible aircraft carrier (granted, their planes are ambhibious). Couple that with the advanced powerplant Fog Fleet has and the fact that 401 doesn't have any planes with her, that means there is enough room and power to store and (barely) operate the Wave Motion Gun.
Despite the US Navy's large presence in WWII, especially in the Pacific, no Fog ships based on American vessels have been seen yet. Why is this so? One possible explanation is that, since America is one of the few (if not only) countries to retain most of its military power and industrial capacity, the American branch of the Fog Fleet is concentrated more in the Eastern Pacific and Western Atlantic, keeping the US Navy at bay. This is confirmed in Episode 11, as a Fog task group made up of American ships arrives to interdict Iona from reaching Hawaii.
Kyouhei: Oh yea, that reminds me. Why are all of you Mental Models girls? Aren't there any guys? Iona: Huh? Kyouhei: No, don't "huh" me. Aren't there any guys? Iona: When you humans leave official records of ship, don't you all use feminine pronouns for ships? Aren't ships female? Kyouhei: Huh? Iona: Don't "huh" me. Kyouhei: (sweat drop) Is that all there is to it? Iona: Yup.
Judging from the imagery from the mental chatroom in episode 11 and the fact that the Corrosion Torpedo hit on 400's sail/fin (thus most likely hitting her right on the control room)... Did Gunzou just obliterated 400's core?
An anime-only instance. There are only six Nagara-class cruisers ever made, and they were all sank during WWII and presumably raised up from the depths by the Fleet of Fog in the intervening years. Earlier on the series, the name ship Nagara was sunk by I-401's crew while defending the SSTO. Cue episode 8, and we see that Kongou's fleet consisted of her, Maya, and six Nagara-class cruisers. So where did that one extra Nagara-class ship blockading Iwojima came from?note While it is possible for the Fleet of Fog to raise back ships recently sunk, it takes a lot of time as Hyuuga in the manga demonstrated. And then you see the fleet pursuing I-401 in Episode 11, which had twenty two Nagaras.
It's possible that most of them were active decoys like the ones I-401 uses. This, however, is not alluded to anywhere in the anime.
Actually, since the Fog don't use the hulks of sunken ships (as the very existence of Hyuga and Haruna prove), this is a non-issue.