No, they didn’t replace the spark plug which now, after your insightful comments seems like it would have been a logical, maybe even critical thing to have done.
Considering all the responses I’ve seen so far, you may be on to something with regard as to whether the ECM is actually the problem. One thing seems clear, if I keep on it, I’ll soon find out and l’ll let y’all know what it turned out to be. Thanks again for your informed observations.