SUCCESS! story follows.
OK, I finally got serious help from an Inuit TurboTax specialist. Seeing my checkered history of clearing this and that, and no obvious hardware or system update shortages, she gave me a fresh copy of TT with a new license code as a download--but I couldn't download it! We saw a "data transfer snag" message and she opted out, saying there was a problem with my laptop or its operating system (MacBook Pro, late 2013, plenty of memory and storage, running MacOS 11.2.3). We signed off cordially.
Well, I had my doubts of course, but a simple ftp file transfer should have caused no problems, so I had no argument to pursue. Hoping that progress had been made, I steered to Apple Support, expecting a Genius Bar appointment with diagnostics run, more software clearing out and reinstallation, and much time spent on all sides. Good luck with that COVID-19 thing. But Genius Bar appointments are almost exclusively for hardware problems these days, and so we tried to narrow this one down by ruling out a software issue. The next step was to reinstall the OS, something I haven't done in ages and was nervous about. I was short of time right then, so we launched the reinstall and signed off. The next day things looked flakey: I could not find this or that function or component location, but today I got another big task done and gathered my courage. I resolved the minor OS problem as a user issue with getting used to default settings I hadn't seen in years, and then easily downloaded the fresh copy of TT, used it to finish my tax returns, and printed a review copy to go over with my wife.
Bottom line: when TurboTax crashed, it was because my OS had somehow gotten corrupted.
