UK cannot land there because they did not own it at the beginning of their turn. US comes after them in the order.
You found the relevant rule and quoted it.
Sub question
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Let’s say there is a German sub in SZ6, along w/a German cruiser. And I want to attack SZ6 w/a British battleship, bomber and 1 FTR. Can the German sub submerse before the British battleship gets a chance to fire on it if the German player so choses?
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Yes, before any of the UK units fire.
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Also note that your british fighters dont get to shoot at the german sub at all, as you are not bringing a destroyer along with you.