You have discovered many of the reasons I refuse to use Apple Photos, and most of them relate to the ways that Apple refuses to support industry standards for metadata. They could, but they don’t, so using Apple Photos starts to feel like a hostage situation. 🙁
To your questions:
- If you export photos from Apple Photos with metadata, the compression is changed dramatically. Generally speaking, the files size will be bloated 2x or decreased by half. I find that to be an unacceptable trade-off to just get a few keywords, and technically it’s completely unnecessary.
- You’re correct that when you export unmodified originals that you lose the metadata you added in Apple Photos. However, I would use this option because most of my clients haven’t added many/any tags with Apple Photos and I prefer to work with the original image captures without any resizing, bloating, or compression.
- I don’t know how to use these sidecar XMP files either. I know a lot about metadata, but I’ve never found any software that can handle these together. I think it’s a proprietary implementation of sidecar files that only works with Apple Photos.
The way I handle these libraries is I copy/export all the unmodified originals and then import them chronologically into Lightroom, using the workflow you’re learning in my Lightroom course. For the “clues” I can always open the original Apple Photos library in Apple Photos to look at things like album names.
Hope that helps!