Well, everything WAS fine until last week... I was cruising down the parkway when there was this sudden single startling thud from the same location that had produced the original problem discussed above. Believe you me, my heart missed a few beats!
Something physically broke loose and went flying out from behind the car. I glimpsed it rolling off the side of the highway, a black chunk of something I couldn't identify at the time, but which turned out to be about 1/8th of the new front flex disk discussed at length above; the thing was unquestionably defective, the cause of ALL the previous problems, and had just disinintegrated. (Fortunately no one was behind me.)
Anyway, the car continued to run alright, though I was understandably very cautious in applying throttle. I crawled to the next exit and called the mechanic from my cellphone. With great care, I took a calculated risk and slowly drove the car to his location via the back roads.
Users of this board need to know that this defective front flex disk was the cheaper after-market version that's alternatively available for purchase online here and elsewhere and not the OEM replacement part.
(I figured it was just a hunk of rubber, so what could go wrong... Live and learn!)
Evaluating the defective old part alongside the new OEM flex disk my mechanic has now installed revealed a significant difference in the stiffness of the two versions. The after-market disk that failed is made of rubber that is significantly less firm than that used in the OEM flex disk. I'd say it's like comparing a tennis ball to a softball in terms of rigidity.
With a new OEM flex disk in place, my mechanic was able to remove those spacers he'd originally inserted in the tranny mount [see above] without any problems. Everything seems to be great now, though I'll post if there are any new developments.
Thanks to everyone who contributed their excellent insights in hopes of helping out here. You guys are the best!