The yoke on the battle car looks fine to me. Given the height, or lack thereof, of the bottom of the cart and the height of an onager's shoulders you'd think there must have been a decent sized bend but there's no obvious way the onagers were connected in any of the depictions I'm aware of, so your interpretation is as good as any!
As for the spearmen, lovely looking figures and differently equipped ranks in a unit are certainly a possibility.
There is some debate over whether only the front rank had shields, but if so the assumption is they had no spears, or whether all rows carried spear and shield.
I'm firmly in the latter camp.
We know there were commanders of 10 (ugala 10) and commanders of 60 (ugala 60) and that units are multiples of 60. I think the Stele of the Vultures shows one such unit. 10 wide, six ranks deep. I believe the six disks on the shields are six bosses on six ranks of shields, each shield having one boss. I assume they were used in much the same way as Mycenaean tower and figure-of-eight shields using a broad strap (telamon) to take the weight and using changes in posture to position the shield.
It would suggest that in each rank there would be an ugala 10. Ugala is often translated as lieutenant but I think corporal would be more accurate for the ugala 10. The ugala 60 would command the whole group equating to a sergeant perhaps. Above the ugalas are the nu banda who are nominally responsible for up to 600 men, these would be captains. In charge of the whole army is the sagina who is the commanding general.
I use the shielded miniatures I have to represent the aga ush, the more permanent troops, and unshielded to represent the erin, the conscripts seconded to the army from the annual levée en masse which also provide men to maintain the irrigation channels, the roads and the like. I have absolutely no evidence for this but it does make it easy to tell which troops are which!
I look forward to seeing more of what you do.