I know the artist rendering showed a 5"/38 Mk12 twin but I could not see an old WW2 gun mount being used or considered.
Actually that's not too unlikely. The Long Beach ended up with one and Danish Peder Skram class had two each. The US was giving them away for free back then and they were still in frontline service with a lot of navies including the USN, so it makes perfect sense that it would be considered.
Sure, but every CLGN-160 design that actually planned for guns from the start used Mark 42. In the missile era, I don't think the USN designed anything destroyer or above with 5/38. Just the DE/DEG, and that's a whole different size/weight/cost class from a proper missile destroyer.
I concur with the eventual fitting of the Oto 127.