Not the same engine but I have a Iveco in one of our farm tractors that is like this, the IP is keyed but the gear it slides onto isn't.
Thanks for the confirmation on the pump key, second time I have heard that now. I have not had mine apart yet to advance the timing, so by just seeing the MW pump on eBay pics, one can clearly see the key slot. Can be deceiving.
The mechanical engine mounted lift pump is supposed to be able to develop upwards of 80-100psi if the output is blocked. The injection pump has an "overflow valve" in it, which is a simple spring loaded ball built into the banjo bolt of the return line fitting. Where the return line comes out of the front of the pump on the outside side surface is where this is located. This holds the fuel pressure in the pump to the 20-30psi range, and then opens and lets excess fuel out. This keep fuel moving through both pumps and keeps them cool, and not "dead-headed" (nowhere for fluid to go). One way to check this OFV (overflow valve) is to pinch the return line with pliers. This will increase pressure inside the injection pump. A shade tree way of cleaning this OFV is to pinch this return line for 5 seconds at idle and release it. It will purge and blow out any yuck that has accumulated in it. RV people with this same engine say this "fixes" 90% of them, and only 10% really need to be replaced, when the OFV is an issue.
I think you confirmed your mechanical pump is working, and your front gear drive is working (PS pump works). Remove one of the large side caps on the governor, and see if the governor internals are rotating with the actual pump shaft. Look up governor spring adjustment under the 6CTA8.3 performance thread as to how to do it, the shut down solenoid has to come off I think. This will confirm the shaft is not broken or the pump driven gear has not become loose.
If it is rotating, the mechanical pump is working, the shut-down lever and it's pinch clamp mount are correct and not stripped, and you tried pinching shut the return line with Vise-Grips, and still no flow, then something in the pump is whacked. It is rare, but can happen. I think there is more of a chance the driven gear became loose. Check the shut-down shaft again, there was a case here where a guy found his to be stripped, but I think his truck would not shut down because of it.
