FWW
Q1: boac is correct. And seeing as within 2-3 minutes after departure, the reading is back to normal, you are in a good position to make an assessment of the reading, and decide if it appears incorrect.
Q2: The Take-Off page, at least on my machine, has an OAT input which I manually key in. But I take the value from the ATIS (or equivalent) rather than the TAT indication. So the machine bases the TO N1 on my input regardless of the TAT indication. I can't speak for the NG FMC.