@SunSurfer I think the issue at play is one of riding style? With a relatively passive riding style, when one simply uses gravity to bend the board, having a shorter stance obviously makes it easier to bend the board, as your experiments show. However, with a very active riding style, where the rider actively uses the legs to bend the board between the feet, a wider stance should afford more leverage to do so, since the lever is inverted.
I experimented with this recently and found that I was able to narrow my stance slightly (54cm->52cm), but that any narrower and my ability to bend the board greatly diminished. I think it may be a two-fold reduction via pure mechanical advantage and also compromised body mechanics as @TWM mentions (I also felt very stiff and locked in place with the narrower stance).
I think I've adopted the (hyper)active riding style out of necessity since the boards I've been riding simply don't bend much underneath me when ridden passively and skid more often than carve when put on edge. However it's really tiring to ride like that all the time and (to me) can feel a little frantic. I've been seeking out a more relaxed ride feel. Now, I think the answer is that I just need a board made to an appropriate flex for my weight.