Just FYI the metadata that gets written to the XMP files do not include the full history of edits that were made on the photo. I was always under the impression that XMP was sort of a backup of all my edits, flags, etc. that would stay with the image no matter where it went, but it turns out this is not the case.
Would all this metadata be embedded inside the image (instead of being written to a sidecar file) if the image itself was a DNG file?
Like, what if when I imported pictures from a memory card into Lightroom I chose “Copy as DNG” instead of simply “Copy”? If the actual picture file was a DNG, would all the information what you mentioned wasn’t stored in the XMP file be embedded within the DNG file, and thus show up when opened in other programs?