OK, I figured out the issue with using PWS: apparently the WUnderground geolookup api returns a variable named ‘tz_long’ instead of the normal ‘tz’ (which is what autocomplete usually returns). I changed the scripts accordingly, and it should now work with PWS as well. When using PWS, make sure you also provide a valid API key, otherwise the query will fail.
Despite lots of rebooting I have only just tried a full-reset. I’m guessing setting the timezone using the buttons during startup hard-codes them, resulting in continued failure to correctly set sunrise/sunset even if the correct data was returned.
I don’t think this is true: the timezone variable is never hard-coded: as soon as the script query returns valid result, it will overwrite the existing timezone variable. So setting the timezone upon startup should not matter.