Definitely not an Engineer or Metallurgist....but having the shaft completely removed from the Gearbox for an inspection certainly makes sense as far as being able to thoroughly examine the entire shaft in detail using advanced techniques beyond a MK I eyeball and flashlight by peering up into the gearbox from one end of the shaft with oil dripping into your eye.
Why if Corrosion is part of the cause for the cracking....why would there be no requirement by EC to Check for Corrosion.
I would think....any corrosion on any part of the aircraft would be of concern despite some corrosion not having the potential to cause loss of the aircraft.
Corrosion on a MGB Shaft seems an issue that would demand evaluation for cause and potential effect upon the serviceability of the Gear Box and thus the Aircraft.