Trouble is if you make it an S/B then you address it as an S/B if it an
AD then its addressed as an AD and has to be compiled with, and if you just put -7 blades on then you get pro rata back from the manufacture.
And not cracking may or may not be via fatigue at all. There may or may not be a problem in the manufacture of the blades.
Simple ground the aircraft with the blades end of story simple and most cost affective way of dealing with the problem