Argh. I had it working last weekend, and now it's beck to the update loop. Has anyone gotten definitive support from TurboTax?

In my case, no. My initial install was from a download.

I am having the same problem on macOS 10.12.6. I followed the uninstall/reinstall procedure, but TurboTax Deluxe 2017 is still stuck downloading the same update...