Whenever I encounter an especially frustrating mystery, I've developed a habit of standing back, taking a deep breath, and looking at the bigger picture. It doesn't matter if it's software systems, trucks, or whatever.
This is because I, as well as many other people, sometimes get my focus locked on one possible part of the problem. Broadening my view to consider other parts often leads to the actual source of the problem, as opposed to what I've been concentrating on.
I've often found putting some space -- in terms of distance and time -- between myself and the problem gives my emotions and mind a much needed break. Looking at or actively thinking about the problem isn't a real break, so avoid doing that. (Engaging in a simple physical activity, such as cleaning, helps divert your attention.)
What I've described above is a skill ... difficult at first and becoming easier with practice.
You can discuss the problem over with other people and read related information. (Surprisingly, less experience people often lead to fresh insights and a different perspective.) Just don't actively engage in the core problem.
During these breaks, I keep a notepad handy to jot down random thoughts that pop up, to be pursued when I do wade back into the mystery.
If the break results in new insights or a fresh perspective, pursue those.
When I'm ready to wade back into the morass, I usually start by taking time to recap the situation. Actually jotting down the recap on paper tends to be most effective. So ...
-- The problem is intermittent.
-- The engine spins but the cylinders don't always fire up.
-- Changing the ICM sometimes causes the engine to fire up and sometimes doesn't.
This is the 'short list' recap, my recaps are usually longer. (If a recap is more than a handwritten page, I'm not thinking clearly and need a longer break.) Also, I'm not there, may not know everything that's relevant to your problem. For example, differences in ambient temperature and interval between starting attempts may be relevant.
Using the limited recap above ..
The starter is getting enough electrical current to crank the engine. The starter uses an incredible portion of available current. So, there may not always be enough left over for the ignition to 'do its thing.'
Something you're doing while changing the ICM may be the variable, rather than to ICM itself.
I'm going to leave it there for now, and close with a related thought.
I really hate seeing people throw money at a problem. This is inefficient and tends to work only if you have lots and lots of money.
I prefer to first find the source of the problem rather than dumping money on all the possible sources. (In the extreme, buying a new motorhome -- off a dealer's lot -- will solve your problem ... this tended to be my mom's approach to vehicle problems.)
However, throwing new parts at a problem can work, if done judiciously. It's important to note inexpensive parts are often the actual source of the problem.
More to come, following this line of thought.
