Can you build a mnr IC on a TT you have just captured or do you have to wait a turn?
You can only place ICs, naval bases or airbases in territories that you started your turn with. So, you would need to wait a turn.
Can the U.S., while still neutral, move to coastal areas of friendlys in the Pacific?
I assume you mean move ships into sea zones adjacent to territories controlled by other Allied powers. If so, then yes.
They cannot however use facilities in those zones, so US ships would not get to use the advantage of the naval base until at war with the axis.
From the Alpha2 rules
The United States Navy, while being neutral, has the following movement restrictions. It can only move to sea zones that are adjacent to US territories in the Atlantic (more specifically Sea zones 101, 89 and 64). US warships (not transports) may also conduct long range patrols into sea zone 102. In the Pacific it can only end naval movements in sea zones that are not adjacent to Japanese-controlled territories and islands.
So in the pacific it only excludes sea zones next to Japanese territories/islands
Here is a great Kill Japan build. Naval Base in Alaska (+ Minor IC if you want).