Wow!! This Site Rocks!
I posted my question less than 24 hours ago and I've already gotten lots of great info. This is simply fantastic! Thank you all!
As a follow-up, I've just sent off Tranny fluid and engine oil to "Insight Services" for an oil analysis. Hoping to find something interesting... I'm keeping the shop at bay until then: they wanted to yank the radiator and start at it; hoping the analysis will detect glycol or other coolant additives either in the motor oil or tranny fluid.
Another mechanic recommended a "block test" in which a sample of coolant is mixed with a diagnostic liquid which turns color in the presence of by-products of exhaust: which would, he said, mean there's a cracked in the block somewhere. Does this sound accurate?