Ah well, I guess we can't accuse someone of malfeasance when they say your part is GOOD and they're wrong, but I suppose we could ask them if they know what they are doing.
Batteries have to be properly loaded test on the right kind of machine, otherwise, they'll fool ya' every time.
On the bright side (?) if you had starter or wiring harness issues, you'd be a lot further in the hole. At least you are to be commended in searching for the right answer and sticking with the problem!
Thanks for getting back to us and letting us know how things turned out---this will undoubtedly help other people who can read about your problem and your investigations.