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This is the first steps of any no start situation. Can you answer these two questions?

Yes, one open slot is correct. 14 pins, 13 wires. When you start the engine, use diagnostics to read some of the data while it is running. Just step through them one by one and record the result. It would be odd to get bad parts over and over, although it does happen. What you really need is parts from a properly operating system. This still sounds much like the Prom failure I had where sometimes just R&R would recover only to fail again.

If the engine won't run then not much can be gleaned from diagnostics except perhaps the TPS setting. It would be good to know of a source for replacement Memcals (Proms) as the supply is pretty thin. The reason I mentioned the integrity of the wiring is because of what I discovered when I took two of the short jumper harnesses from the front of my 1990 LN3 and a 1995 S/C engine. Both had failing loom and as long as they were on the engine stand I pulled them out and removed the loom for replacement. In both cases, I found multiple points where the coating on the actual wires were flaky and sometimes bare for an inch or more. I don't know how long the damage existed or if it was magnified by taking the harness apart, but when two wires that are bare in the same approximate location and are parallel to each other, it gives pause. It is a short harness that connects the knock sensor, cam sensor and crank sensor to the ICM. It connects to the main harness from the firewall on top of the engine just behind the power steering pump. Looking for not only continuity, but also a short to another wire in the harness. Edit: I think it is only the cam and crank sensor connection to the ICM. It is an eight pin plug behind the power steering pump.

This is just a shot in the dark but something worth checking... The only logical reason I can think of that the engine will start with the old ICM, and not with the new one, is maybe one of the pins in the ICM connector is broken off and sticking in the old ICM. That might allow the broken pin to make a connection with the old ICM plugged in but not with the new one.

When the engine will not start with the new ICM are the spark plugs getting spark from the coils? That is the first thing you need to know. Next thing you need to know is if the injectors are spraying fuel. Knowing the answer to those questions will give you an idea about where you need to start looking to find out why the engine won't start. If the engine will start with the old ICM and not with the ones, I don't see how the ECM could be the problem. The ECM has no way of knowing which ICM is connected to it. If you do the test properly that I recommended, I'm confident that you will find the problem.

Im bolting it down snuggly.

I have to wonder why you are trying to get an answer "over there" when they same guys you are getting help from here are also over there.

"R" codes?? You are looking for "e" prefix codes, and "c" or "h" suffix to indicate if active or not.

One thing I forgot to mention about the inexpensive spark checkers: If you have one or more that appear to indicate irregular spark, switch it with another and observe if the problem child follows the spark tester. The way the ignition system works, if you lose one spark plug, the other one of the pair is likely compromised as well.