1) Why is there such a limitation for the starter in the first place ?
The starter mechanically disengages from the engine above a certain speed. If you re-engage it above this speed then it won't spin the engine. More critically, as the engine slows down the starter will re-engage while running at its maximum speed and shear out.
You have the higher N2 in flight probably because at certain speeds you'd find that you wouldn't be able to attempt a start, but the engine isn't running fast enough for an in-flight start.