Well, that was stupid easy.
Once I set "Browser 1" in the Mac client, everything else went through without any issue. Logged in with my Free Trial account, first run cinematic, character creation, introductory quests... no problem.
I did find a link in the launcher: https://support.na.square-enix.com/faqarticle.php?id=5382&la=1&kid=67598&ret=faq ...which specifically states "The Mac version is not compatible with the latest launcher theme. Currently, the launcher will only open in its old theme for the Mac version." There you go.
With regards to CX, after the install of a couple hundred dependencies, the first run of the launcher/patcher generated "An unknown error occurred in register_dlls" message. I clicked Skip and let the process finish. I started the Launcher, it apparently fixed the error and I got to the old theme login window. (Apparently, I left a .cfg file behind with the Browser 1 setting). I logged in with my windows-licensed account and am now facing four hours worth of download/patch for Shadowbringers
Overall, other than one-time oddities, everything works as expected. The only issue I found is that I cannot use the Tab key to move from the Username box to the Password box. I can live with that. :)