This is not an excuse for the shingle disintegration, but a clue as to the proper dynamics of exhaust ventilation.
Firstly, by having more than one type of exhaust syetem in place; ie; Ridge Vent, 2 Power Attic Ventilators and the Gable Wall Exhaust Vents, the entire air flowage has been “short-circuited.”
Think of it this way. When one of the power attic ventilators kicks on, where does it achieve its air supply from?
The path of least resistance, should be concluded.
Therefore, since the air flowage is being short-circuited in multiple manners and locations, the under soffit fresh air intake vents are never supplying any, or at least minimally, the amount of fresh unheated and non-moisture from interior humidity levels, laden air into the attic.
That, most likely would be the shingle manufacturers claim to reduce liability.
Secondly, on the rafter insulation, you stated that there was a minimum of a 2" space for air flowage and that it was unblocked. Was this due to them placing insulation baffle vents in place continuously on the top side of the insulation? Or, is this due to you observing an approximate 2" sag bowing downward in the center of the insulation between each rafter? Even if there were a 2" sag bowing downward, can you be absolutely sure that the sag is 100 % continuous, from each soffit vent all of the way up to the continuous ridge vent?
Thirdly, are there soffit vents in EVERY single rafter bay? Even if there are a substantial number of them in place, they must provide fresh air intake ventilation for every single individual rafter bay to achieve ventilation for that portion of the attic and that individual rafter bay pathway.
Now, I have never, ever been a proponent of the IKO brand of shingles. From a former IKO Representative, who now heads an ABC Roofing Supply division, I was informed around the early 1990’s, that IKO was flooding the US market with inferior quality product at a bargain basement price, just to achieve a recognizable presence.
That is just hear-say, but I have seen many instances of poor quality IKO product failures compared to most other brands being sold at that time period. Globe was another brand at that same time period who allowed their product to become inferior, prior to them going out of business.
If the ventilation aspects I spoke about earlier have merit, I would presume that the deck sheathing has suffered severe delamination as well. Possibly, if all of the decking was replaced at the same time as the roof shingles, it may not have been so negatively compromised and noticeable as of yet. But, there should be significant signs of humidity and/or moisture leaching into the plies of cdx sheathing at a minimum.