Let me know if you need anything more or would like to test anything. One of the OSPis is spare for a few more days so I can break things on it without plants dying.
For the timezone, maybe the system timezone could be returned when queried by the mobile app, etc.
I am glad to hear the options setting is being fazed out, as a sysadmin it’s bugged me from the start as being a possible place of conflicting and poorer quality data… 😉
And lastly, maybe a bug report could be filed with the maintainers of raspi-config to have a record of it having been run and what has or has not been set would be very useful. Then you could query as to whether the timezone and locale have been set and if not show a set of instructions on how to fix it. The same would probably apply to other projects for appropriate settings like the camera, GPU memory split, etc.