I should imagine that compressor speed limitations in decreasing air density may be the limiting factor. With increasing altitude, the collective pitch setting will be higher for a given airspeed than at a lower one, thus requiring more engine power to counter increased drag. OEI the engine may be unable to achieve the compression needed to match the torque produced during AEO flight at a high density altitude, without reaching a design limitation. Same thing happens with the generators.
So the answer to 2 is likely yes, you will be limited by Ng and that will dictate the PI value.
Answer to 3 is no for 2.5 mins and yes after that. The EECs will PI limit to whichever limiting factor comes first (Tq, ITT or Ng), and not allow more than the 2,5 rating to be delivered. But once that limit approaches expiry, the pilot has to reduce the power requirement.