As I understand it, by the time that all North Sea EC225 operations resume, every shaft will have been examined minutely not only by the techniques you mention but also by very close engineering inspection of the inside of shafts for any hint of "mud".
Some operators have said that they have found shafts with sludge. The process calls for cleaning the sludge, then an NDI. Isn't this is done with the shaft in situ? I am not aware of the requirement to remove the shaft and inspect it under magnification for any corrosion (I would love to be corrected) My point is that if you have sludge and you don't then inspect the shaft under magnification, you could have corrosion? Therefore it is the NDI and the MOD 45 which are the safety barriers.