By utilizing the gable end vents as an intake vent, you would be denying the entire attic cavity from being completely “washed” from the lowest point, where the fresh air intake ventilation inlets are supposed to be positioned.
Ed
Re: Code. Yes, there is a 1/300 and a 1/150 formula, which then creates code compliance, but if it is a design and functional issue, more attention deserves to be expanded upon, even if that means to defer to a ventilation “expert”, if there truly are such an animal in the roofing industry.
Most everything only gets proven after years of real world application versus theory and hypothesis. Most of the “Scientific” studies take place in model like, un-real world situations.
Google: Short circuiting attic ventilation, for a fuller understanding of this “hypothesis”.
I also agree, that the insulation on the bottom side of the decking seems to me to be the primary culprit for the "over-heating of the shingles premature decay in addition to the brand of them and in addition to the "short-circuiting of the ventilation flowage.
Ed