I finally found a way to get Final Fantasy 14 working again thanks to a GitHub gist. The author essentially replaces the Final Fantasy 14 Protonfixes launcher script with one that installs FFXIVLauncher into the same Wine instance Steam uses to invoke the game itself. It then uses this launcher instead of the broken one Square-Enix demands. Since Steam invokes the script within Proton, the replacement launcher passes the expected steam account ID to Square and everything magically works.
Final-est Fantasy Last weekend, I went to try and play Final Fantasy 14 like I normally do, and ran into a nasty surprise. I’ve been playing on Linux this entire time after jumping through a few hoops, and while the game works just fine, the launcher is another matter. Apparently some of the workarounds I put in place were to institute the “old” launcher because the new one uses MSHTML of all things.