AHA! Once again I owe a debt of gratitude to the RC Community. Went out & got some contact cleaner & tried that, but still couldn't get past the calibration failure at the third point (upper right). So, figuring at this point that I had very little else to lose, I pricked the contacts several times each with a needle sharp awl, and....
after one more failed calibration attempt, it completed calibration and is now back among the living. Downloaded my .pcf back into the Pronto and all is happy. It's now running 3.9.8 firmware, and I'm not too eager to try upgrading at this point - somthing about leaving well enough alone.

Thanks again to all who contributed to this massive thread, and especially those who took matters into their own hands to find a solution rather than depend on the oxymoron that seems to be Philips' idea of customer support. Oh, and a special thanks to Philips for making such a wonderful product and doing such an exemplary job of supporting it...NOT!