Like you said there's 2 different theories on it. 1) Quads aren't strong enough, allowing the relatively stronger PC to compensate via a goodmorning movement or 2)hamstrings are not strong enough, cant hold back angle, give up and allow hips to shoot up with the squat being completed by the quads. Which of these is true? Maybe neither.
The squat is a full body movement. If some part of the movement is affected by a weak point in your physique or strength distribution then you should get stronger there, wherever there is doesn't matter, only that you're squatting correctly at a given weight and progressing up from there while maintaining correct form. It could be anything, upper back, abs, erectors, who knows. Maybe a bit from column A and a bit from column B, ..., and a bit from Z. You nailed the causality: "however since i can sit rock bottom in a narrow grip overhead squat and only lose my upright torso on the way up suggests it's not my problem." - this rules out mobility and suggests a strength weakness. When the weight is lighter all of the muscles in your body are more than capable of lifting the weight correctly. As you add more weight, the leverages of the lift change and the demands on the various muscles also change (not necessarily proportionally!) which places more of a burden on some muscles to compensate for a weakness in others, hence departing from textbook form.
My favourite theory is that form has a strength component to it, if you are strong enough to maintain good position throughout the movement of a backsquat in lighter weights but can't do it as it gets heavier, it means at a certain point the weight overwhelms that weakpoint. Identifying where that occurs and grooving the movement just below that weight will allow you to overcome that weakness and improve your form beyond that weight.
If I was going to fix someone's GM squat .. i'd have them work up to a heavyish single, then drop down and do volume at just below their form breakdown weight, working to increase the weight steadily over time but being patient before adding weight. Maybe master it for volume (say 6x6) before incrementing and repeating the process. What i wouldn't do is do a "normal" squat workout and then do technique work with lighter weight -- at that point fatigue has already set in and you're going to have bad form even with lighter weight.