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.
Why does Haruna always seem so meek and feeble whenever she loses her coat, as opposed to the indomitable mountain of a person she usually is with it? Don't forget, that coat is pure nanomaterial, which, unlike other nanomaterial clothes, is fully capable of tanking hits from infantry! She probably feels far more naked without it than one would normally assume, because it's a shield to protect her body, as well as her modesty.
Also the fact that, well, she doesn't really wear much under it to begin with until later into the story.
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?
In the anime, Takao sacrifices her physical body to reconstruct the I-401. However, considering she still retains her core and processing power, there should be nothing that prevents her from reconstructing a physical body. It wouldn't take a lot of nanomaterial for that, and the Ars Nova should have at least that little extra to spare.
Why not just go to China? Seriously, it's a fraction of the distance, and it's got a land connection with India, Russia, Europe and Africa. Several decades is more than enough time to build up some pretty impressive overland trade routes.