Thanks, MrC.
When I import on the Mac side via MPL, there is no dialog to select file types in the import sequence. MC just asks me to select the MPL and when I navigate to it in the file browser and select it, MC immediately begins importing files. (BTW, I found the culprit on the one failing track--missing comma in fllename tag. So now all 2240 tracks import via MPL). So as the user I don't know what if any file-type settings are used.
When I import via Tools -> Import -> Import a single folder, it doesn't seem to matter whether I only check DSD file type or allow several other file types. I still only get 307 files from 28 albums. And like henry2 in a thread near this one, a few of the albums import all tracks with all the tag fields populated, but many only import one or two tracks from an album, most often with missing tags (the occasional "unknown album," or just several blank fields--which *are* populated in the media files--I trust--and the MPL--I know). And 1933 tracks from 82 albums (including embedded cover image files) are not found/imported at all.
This suggests that MC cannot read the metadata saved in the media files without guidance from the MPL. I don't understand this, since the MPL is generated from tags saved to the files and back to the library, many of which were created using the MC tagger. Is it possible the DSF files cannot retain new tag data because of some frame error or lack of standards, and each time MC imports the files via MPL it has to read the tag info back into the imported files? But surely it doesn't need the tags to read the path to import the files, even if it's missing metadata? Why don't I have 1933 more tracks imported, but with empty or corrupt tag fields, when using 'Import a single folder'? (I should say also that the Oppo recognizes these all as DSF files, according to its file-type indicators when it plays them.)
If I use File -> Open Media File, MC will navigate to the file on the HD and will play (through my laptop speakers) any of the files I've randomly selected, including ones that don't import using "Import a single folder." So obviously MC can read the file paths if I use the file browser in this command sequence.
And why can my Windows colleague import all but 24 of the 2240 files? I haven't yet learned whether he sees all the tags, but will pursue that. That's really the most urgent part of my question--why are those 24 tracks failing on the Windows side? My Windows colleagues are not used to importing via MPLs and don't want to bother with them if avoidable. I don't mind for my own use, if that's the only reliable method, but I presume the MC Mac team wants both methods to work equally well for DSF files.
I noticed when tagging that MC's tagger seems not to recognize the option-key combinations for diacriticals when entering tag values (no Götterdämmerung) but if I create them in the MPL via an editor, MC displays them correctly on import (via MPL), as it does if they were part of the metadata present in the original ISO. That would be a good feature to correct. The Oppo displays them correctly as well.
So I thought perhaps the import failures might be related to this, but among the 307 files that successfully imported via "Import a single folder" are several with é accents and ü or ö umlauts, so I presume the character set isn't relevant, at least to finding the full filename/pathname, since many more than 307 files are free of special characters in the pathnames.
It's a head-scratcher. I hope it's some user error on my part, since that would be easy to fix once known, but I fear it's something deeper.
Thanks again for thinking about this.