Bushings: Excellent! Someone was thinking.

Pin: Technically, even with no diagonal cable, you are right that the pin is loaded in shear between the closely-aligned hinge fingers. However, under typical rider load with the top of the hinge halves pushed together and the bottom of the hinge pulled apart, the span between those two is small, so the shear forces are high. Or at least a whole lot higher than with a triangulating cable; In that case, the bending load is spread over the vertical span between the hinge, and the bottom bracket where the cable attaches, so less force at each, and the pin is loaded in the shear force compressing the hinge, at a much lower level, like 2-3 times at least, looking at my bifold in front of me with a triangulating cable. Also, by the forces then pushing the hinge together both top and bottom, there no increase in load on the hinge latch other than the static preload on it as the latch goes "over-center", snapping into place. And, under front braking, with no cable, the pin shear loads reverse, whereas with cable, the hinge loads stay compressive shear, I know this because the cable never goes slack under braking. Almost 2 years on my triangulating cable, no stretch, and zero adjustment on hinge, whereas before it was monthly. Hinge just stays tight, no slack, and I really feel the difference when climbing out of the saddle, the torsional stiffness of my frame feels like my Cannondale racer, instead of like a bifold, which it did before.

The hinge on a Bromption, because a lot closer to the heat tube, should not be loaded in bending as much, though just as much torsional loads. But I see no reason why a triangulating cable would not have the same benefits. Oh... yeah... that would most probably interfere with the rear triangle swinging up inderneath when folding. On my cable, not bolted on but wrapping around the head tube and BB shell, when the main beam folds, the cable goes slack, and I could just slip it off the BB shell and over the left crank arm, if I needed to get it out of the way. So could do same on Brompton.