I can't say the Restart is definitely the answer, but it seems to me that the restart should happen after the reinstall and the first update have been done.Now ...

What finally seems to have done it is to insert an additional step between 2 and 3 in this sequence: <a rel="nofollow" target="_blank" href="https://ttlc.intuit...

I followed the above linked instructions but it just keeps downloading the same 220.5 Mb update.What next?

Restarting the Mac didn't help. I will now try uninstalling and reinstalling TurboTax and all its associated files, including the State file.