Yes he would!
We've gotten arcs that stayed underwater, like Stan Lee's the Quest and the Harras and Lee Suma-Ket arc. Roy Thomas would do stories underwater, but it was definitely interspersed with forays on the surface world. Alot of the DeMateis mini-series was underwater, but it was all motivated by an airbreather. But yes, Marvel definitely believes that Namor HAS to mostly interact with the surface world or readers won't be interested. Which, IMO, is entirely bogus.
I didn't think about that, but that's true. We don't spend enough time in Atlantis to get any details.
This! That's why I've said, the MCU Namor isn't going to be Namor if he isn't bi-racial, and the perpetual outsider.