The original problem that you described is likely one of lubrication.
You stated that the shutter plunger was at a comfort distance from the shutter button on the camera, when you first installed the grip, but that it did not retract all the way after the first ( or subsequent) triggers.
I'd suggest that you clean the cable link between the thumb-trigger and the shutter plunger, and lubricate it with a dry lubricant, such as is used for bicycle cables.
Reassemble the unit to the original configuration ( as in the photo that you included) and exercise the unit a bit before re-attaching it.
MAMIYA designed the unit well, and you do not need to re-design their assembly.
I wish what you said is true. I have two RB67 Pro SD bodies in near new condition or hardly used before I got them. I also have two multiangle grips in similarly great condition. Just by looking at both I see significant discrepancy how each lines up with camera body. So no, Mamiya did not do as well as they should have.
One of these grips had screw the other way around (from what apparently Mamiya is showing in their literature) and that's the one I've always used and with no trouble. Second one, with seemingly correct way on that screw did not engauge with the button correctly, just like in OPs case. And getting more picky on Mamiya's QC here, the trigger plates of the grip that sits in front of the shutter button are not lining up the same way. While there is an intended elongated slot to make the adjustment right so they face each other as they should, the discrepancy in the line up and the difference in adjustment each requires only means it was not so swell at the engineering level. And no, neither grip is showing any signs of having been dropped or any part bent out of shape. They look as pristine as one would have ever hoped for. I gather experiences will very for many users on this issue, which only further questions how they came out of the factory.
These grips are fine, nearly solid and I like them better then fixed ones as they can be adjusted quickly for different situations. But they do have some play in the pivot pin so not as rigid as I would have liked.