It's not been mentioned whether Nagumo has that information, UT. Fleet Command (Yamamoto on the Yamato) has decided that it was the 3rd US carrier the sub spotted, that the Transport fleet had likely been found & ordered them to join up with the Invasion fleet for support.
Doubtless if the sub doesn't answer the call for clarification within 6 hours or so, they might reassess the situation & conduct a more thorough search. What they'll make of it if they find all of Force Z is unknown, although they'll probably dismiss it as an error to begin with.
See above, Derek. They don't think it's an escort carrier.
By this time, German subs could send & receive radio messages on certain frequencies while submerged but above 50 feet or so, Dathi. I can't confirm that the Japanese had that ability, but it's reasonable to assume that the Germans passed that information along, so maybe they could too.
As for reporting; I'd say if the sub didn't respond to the message within 6 to 9 hours, it would be regarded as suspicious. Anyone have a better estimate?