I'm guessing Patty is the one to answer this accurately. I have only personally seen a few(bad) egs which looked like they were done by people who believed horses should have square feet! One farrier I met who preached 'Natural Balance'(bite your tongue on that one Patty!
) actually argued with me that all horses have 4 'pillars' on each foot(inc hinds), being heel corners and precisely at 10 & 2o'clock on every foot of every horse...
I personally feel that it is just one more observation that's worth understanding & considering, but that individual horses, postures, etc, etc mean that, as with most things, it's not relevant or same for all & so not something to force onto a foot. I also didn't think it was about peripheral loading, but it was a long time ago I looked into it & perhaps I'm letting more recent info colour my memory on that note.
At any rate, I don't believe in 'peripheral loading' - that is, the hoof wall is the primary weightbearing structure - but that the *internal* wall shares some of the load. And generally speaking, in unshod hooves, quarter walls shouldn't really be loaded. For application of a shoe, the farrier must trim the hoof flat on the ground surface, to best distribute weight around the walls, but bare hooves tend to be trimmed to the sole plane, which naturally tends to have an arch at the quarters. Peripheral loading tends to promote flaring & separation, especially at the quarters, if they're left trimmed to be flat.
So... based on what you have said, sounds like your horse's quarters have been too long & have started to separate, which was allowing stones to get stuck there. So, whatever lable you want to give it, sounds like I would indeed 'scoop' those quarters, as much or as little as the sole dictates, and due to the separation, would probably bevel them pretty strongly.
Yes, the hoof flexes, but more to the point, it's not just the walls but more so the frog & to a degree, the sole which is bearing the load. Also consider how much work/life the horse spends on hard, flat surfaces(where bare or shod, the horse may be peripherally loaded to some degree) or on yielding footing, where regardless of trimming, the entire base of the foot is supported & supporting the load.