unknown keys when looking up photo:getDevelopSettings(keyname), etc.

The most robust way, but also the most time-consuming, is to construct a dictionary of all the keys, using standard reverse engineering:

- For getDevelopSettings(), take a test photo and set all of the options and sliders in the Develop panel to non-default positions, then dump the result of getDevelopSettings().

- For get*Metadata(), start with the documented fields and write a test program that calls get*Metadata() on each field in turn to validate it. Then in the Metadata panel, for each tagset preset, fill in each field with a test value.  Then call getRawMetadata (nil) and getFormattedMetadata (nil) to dump out the keys, verifying each field value in the panel has been returned.  (In past versions, get*Metadata (nil) didn't always return all the keys, but that may be fixed now.)

Constructing a dictionary ahead of time ensures you've got the best understanding of all possible keys,  A less time-consuming and less robust way is simply to catch errors using pcall:

local success, result = LrTasks.pcall (function ()

return photo:getRawMetadata (...) end)

This approach is less robust because it can mask other sorts of errors, not just invalid keys.  But it may be the expedient choice.