Hi salamanderpres
If there was a considerable IRS drift at the top of descent, say 10 deg for instance, this would gradually get corrected as the aircraft comes down the beam.
Afraid not. The FMS position will get updated and calculate a position bias to be added to the IRS position. It will interpret the difference between heading and track as a wind vector.
Airbus wants the check at 350 ft,
because the latest you can Arm Approach for A/L is 400ft, therefore 350ft is your last chance to check it. FCOM 1.22.30 P50:
Where did 5 degs come from? It's not published in any of my manuals.