I'm somewhat confused by some of your descriptions of how you are performing the Ohm test. To confirm that you Ohm'ed the injector returns all the way to the PCM right? Performing the test with the main engine plug undone doesn't test the wire path all the way back to the PCM.
Ohm tests are often the first tests used because they are straight forward and easy to understand. IMO it might also be important to understand the limitations of the test methods currently being used. Ohm tests don't always find every kind of wiring harness fault. Remember one of the conditions of an Ohm test is for the circuit to be inactive. However the circuits is not used that way. It is used with current flow occurring in many different circuits.
I'm a big believer in Occam's Razor. Occam's Razor would have us to look for a straight forward cause to this problem. I'm sure once the problem is found it will make sense.
https://en.wikipedia.org/wiki/Occam's_razor
Many times when I work on a serious head scratching problem that simply doesn't make sense I find it helpful to stop and reset. Go back and double check some of the BASE assumptions that started the trouble shooting session. I'm assuming that you bought the car with it's current problems. So some parts of the car's history are unknown. Recheck everything. Take nothing for granted.
For example. I remember helping someone with a wiring problem that made it appear the entire car's wiring was F'ed up. To make a long story short the problem turned out to be two similar large connectors were installed backwards.
I have also helped people with electrical problems caused by mixing and matching wiring harness parts from different model year Mustangs. Not realizing that Ford uses the same connectors that LOOK the same but how they are wired are very different.
We know for a fact there's a ground on CCRM pin #17 but the wire path tests good right? Where is that ground coming from? IF the signal return is not grounded external to the PCM then the ground has to be coming from the PCM right? What might cause this on two PCM's?
Just trying to think outside of the box. What if the wrong PCM has been used by the prior owner? And when looking for a replacement you matched the numbers on the PCM installed thus installing another incorrect PCM?
Another possible theory is there's some external problem in the wiring harness such that it shorts out the PCM when turned on. The new PCM suffers the same fate as the original. Have you inspected the printed circuit board of the new PCM looking for evidence of heat damage?
Unless you can find an evidence of other work done/swapped/repaired on this car you may be in for a long row to hoe. It may be necessary to test virtually all of the signal lines back to the PCM. In addition to testing the wire path you should also double check that the pin outs and wiring colors match the Ford Wiring diagrams. Again. Take nothing at face value.
I think it's official. You have a real puzzler on your hand here.