Okay, SOLVED issue and updating here for those following this thread. P0305 code became very frequent, and finally started to have P0300 multiple cylinder and crank sensor code. Probably unrelated other than continued electronic deterioration. Decided to replace crank sensor, and P0305 code disappeared, but P0300 continued along with rough running. Decided to purchase NEW coils and ICM as a final solution, even though had previously tested and exchanged these from another used loaner vehicle. Purchased stock coils and ICM from O'Reilly Auto, their lowest cost versions. Immediately the vehicle began running better, probably best in years. Have over 2,000 miles since, and everything is perfect. None of the test equipment or swapping with used components would solve the issue, but the new coils, crank sensor, and ICM solved it.
Previously in my career I worked with electronics manufacturing/troubleshooting, and most likely there must have been a general deterioration in the electronics, and individually the components may have been close to tolerance and tested okay, however when put in line together were causing an issue. I know this may not make sense. In the industry we called it the stacking of tolerances, where if the components inline were near/at the tolerance limit, the end result could be erratic. I've had this happen with our company on computer diagnostics, where a processor may not work in one computer, however when placed in another identical spec computer (motherboard), it would work fine.
So, short story I believe was a general deterioration in the electronic iginition components, and by replacing with new the issue was solved. When replaced 1 at a time, the issue was not solved, but replacing all inline solved the issue. Probably a one-in-a-million issue which made the diagnosis so difficult.