Hi again DrChris,
Looking back and rereading your initial post, it seems that in order to fix this problem we are going to have to start eliminating the objects in the equation.
We will assume from your posts that there is no restriction from the pump to the multiport valve (the evidence there is that your backwash pressure drops to <10 psi). That leaves everything after the multiport valve as a suspect, which are filter, valves, pipe, heater and return eyeballs.
You stated that the lines have been scoped and there are no blockages. From that we can assume that the valves were checked also when they ran the scope through the line. At this point, i will have to make the assumption that all the lines were scoped from the filter to the pool. If they were not, that is something to consider.
So what does that leave left as suspect, the filter, heater and eyeballs.
You have ran extensive back washes that should have flushed any debris out of the filter that should be blocking it. The possibility of something left in the filter after back washing is very slim. You have posted that you ran the pump on recirculate and the pressure stays the same (~30 psi). On recirculate, the water is bypassing the filter completely so that will rule out the filter (if the filter was the problem, the psi would have dropped to <10 psi at that moment).
The eyeballs have been taken out and found to be clear and free of any obstruction, so that rules them out.
I think Mark (Mas985, in post #17) is on to something. By ruling out everything else all we have left is the heater.