"can't assign artillery"

[PlasmaKrab]

I don't know if you already checked this up, but only formation leader units (0 units) can assign artillery, as opposed to any other unit in the platoon. So if the X0 unit of your scout platoon is one of the onboard scouts, you should unload it first to plot the fire mission. Afterwards you can reload it, knowing that it will lose eyesight on the target and the bombardment will lose accuracy.
Reversely, you can tweak your roster in the deploy phase to make one of the LAVs the platoon HQ.

Apart from that, I'd be interested in getting a look at this scenario once you are done with it