OK, so this may be the final post to this thread.
I want to thank everyone for their help!
I've finally found the issue and corrected the situation!
In case someone else behind me has this issue, I'll describe what I was doing when the non-start condition occurred and what I did to remedy...
I was working a master cylinder replacement.
I was having issues with getting the new Master cylinder to prime, so I had the MC in and out multiple times. This is when the non-start occurred. In pulling the MC in and out, I must have inadvertantly yanked a wiring connection out...
After multiple starter circuit checks, I found a thick yellow wire under the front bonnett near the nose of the MC, that had become unplugged from it's connector.
Plugged it in, no more problem, she starts fine again.
So now I've got a new master cylinder and a new starter. As it turns out, the old starter was dirty and old, but still bench tests fine. I'm just keeping the new starter in the car, since all I know is the old starter is way over four years old...
Thanks again for all your help, and cheers!

BTW, if your new master cylinder won't prime and you're at your wits end, you may need to prime the Master cylinder with the booster connected, outside the car. I spent approx 15 hours with this project! In the end, it actually was a 2 hour project, but becasue nobody I've seen or heard of has had to prime the MC with the booster attached, outside the car before installation and gaining prime, I had a lot of learning curve here... I'm happy to post this here in hopes of helping some poor soul from wasting as much time as I did...

You said several times you had voltage to the yellow wire at the starter. How is this possible if the wire was disconnected at the MC?

FYI: there is also a plug under the dash that can shake loose and cause this.