Well, my main computer is back and on-line!! Yeah!!!
So. you ask, what was the cause? Well, when I bought the computer last fall, I bought a memory upgrade (from 8GB to 32GB) from a 3rd party OEM supplier for Apple that I had bought from for previous computers. In the end, it turns out the root cause of all my problems was that one of the four chips went bad. As a result, when I ran the one patch I tried, it corrupted the install and hence all the other problems.
As I noted, I was fortunately to find a local Apple retailer and service shop that was still taking appointments. I had them do a data recovery (since I wasn't sure what they would find) as a precaution. That is what took the most time. I had about 1.04TB of data to recover. And yes, they got it ALL!!! (which was my biggest concern). Once they got past that, they ran full diagnostics and found the faulty memory. They replaced the memory with new chips (which they also warranty!) and restored the operating system, and reapplied the data.
An expensive lesson learned, but they have definitely earned my business in the future.
