Perhaps bending moment is a better term.
You're saying that in all of these squat variants, there is no involvement of the spinal erectors or the core muscles, that the load is purely compressive and acts vertically down the spine?
No, Every muscle under the bar is working mechanically and supportive.
If the spine maintains its natural curvature throughout the movement, then there can be no "shearing force" of the spine.
The main issue with squatting and the most important in relation to spine integrity is ones ability to squat deep without any posterior pelvic tilt.
A classic example of short torso and long thigh and legs, will need to lean forward more, placing the bar lower on the back will make matters worse.
Shearing force is best described when a trainee while using a leg-press, places his legs too low on the plate.
Where the line of force is pulling the joints apart rather pushing together.
Joints being pulled apart, places the emphasis on tendons and ligament trying to keep the joints together, and little emphasis on the actual muscles performing the mechinal work, decreasing any chance of improved flexibility, which is one of the many elements/byproducts of strength.