I see all your points. The pinion angle would have to rotate down as the axle goes up because the bar attached on top of the axle is shorter, so the top bracket would get pulled inward as it passes over the midpoint of that curve.
BUT, the part I'm still confused about is how it works with the top bar coming into the axle at an angle. Now the plane of this radius goes through the frame at about a 30 degree angle. So if you imagine the axle travelling with ONLY one diagonal bar attached, and also imagine it blown way out of proportion, the axle would get pulled to the side along this angled plane as it travels.
So with both trianglulated bars attached, one is pulling to one side and the other is pulling to the other side, obviously with both attached it can't go either way, but doesn't this cause bind in the links as those pivots try and move along their angled plane and the axle only moves in a plane parallel to the frame?