The potential problem I am describing has nothing to do with suspension travel or side-to-side movement at the Heims. It has to do with the limited resistance to rotation about the axis between the upper and lower Heims at the narrow ends of the A-arms. Given mechanical clearances and elasticity, the narrow spacing of the upper Heim pair cannot be considered a rigid connection in a steering rotation sense, but rather just an upper pivot point. The front wheel assembly is effectively mounted on a kingpin with dubious steering rotation stiffness.
IO,
Giving consideration that you are making a valid point that I'm just not understanding in the context of your argument, let me provide some illustrations to better clarify what I am thinking when I read your points. The number #10 image below shows your description of "rotation about the axis between the upper and lower Heims at the narrow end of the A-arms". The 3 green directional arrows show the axis I believe you are describing since you said side to side movement is not in question. The two yellow components are the spindles centered on the top and bottom king pins affixed to what I describe as the ?steering knuckle?. If that term is foreign to you what would you like to call it? The vertical and horizontal black lines represent the center lines of the spindles through the king pins and center axis.
When you say "the front wheel assembly is effectively mounted on a kingpin with dubious steering rotational stiffness" are you not describing virtually all common steering/spindle/kingpin configurations whether mounted to a-arms or bosses welded to solid axles? Even struts act as the spindle where the top and bottom connections swivel about a centerline which serves the kingpin role. Would the single connection point of the kingpin at an a-frame also be considered dubious based on your description here? Why would two heim connection points be any more dubious than the one common ball joint connection at an a-frame?
Where is the "dubious steering rotational stiffness" you are describing? Is it the king pin mounting bosses coming off the steering knuckle? Are you saying those points have movement because the knuckle itself is held in place by the a-arms at their two heim connecting point? Exactly where is the center and plane of the dubious rotational axis? If its the plane represented by the 3 green arrows? Where does the dubious rotation come into play?
So, lets say we have no front suspension and the steering knuckle is solid mounted to the frame as represented in attachment #11. Here the knuckle is mounted at the same 4 flanges, two top and two bottom. Do you still see dubious rotational stiffness when the spindles are steered left and right through the tie rod?
Attachment #12 shows the 6 degree extent of spindle movement to the right and inversely the same to the left. Because this is a zero scrub radius steering setup I don't see the steering load on the steering knuckle which is a 110mm x 40mm cross section x 420mm diameter solid chunk of metal as a dubious component. Neither do I see the connection points of the a-frames as questionable. I might believe the a-frames themselves could be beefier but considering the low impact application of running up and down the salt I don't think so. But as always I could be wrong. The yellow spindles will likely need to be beefier in real use but I'm not trying to engineer structural integrity in these drawings. This is currently just a concept exercise. Please help me to understand if you can what I am missing about your argument. Thanks... Terry