About the unique image ID thing: One solution that could work would be to use a unique number and write it in all files - and perhaps add a f0f0f0 (or something like that) to the original file:

• 01234567890123456789f0f0f0: the original file
• 01234567890123456789: any of the derivative files

For the unique number, you would either need some kind of database - or any other way to provide unique/consecutive numbers. You could, for example, calculate the hash of the original file - be it SHA1, SHA-256 or even MD5 - and then write that hash into whatever EXIF-/IPTC-/XMP-field you like.

Note that if you change some none-sidecar data (EXIF, IPTC) later on, the hash will change, too - so therefore, it would be best too calculate it as soon as possible, or at least it requires you to know that re-calculating the unique field later on (e.g. because you accidentally deleted it) will lead to a new hash.

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As to what field would be best: I do not think there is a definitive answer. Write it to any of the fields that is prominent in all the programs you use at the moment - and perhaps in some of the not-so prominent ones, too. That way, you can always copy it to some other fields later on if needed, and also, you have a backup of the actual data if something bad happens to one field.

I would take one string-able field of each supplied metadata-standard - so one for EXIF, one for IPTC and optionally, one for XMP.