I definately think it's iTunes that is changing the modified date, but I think it's because of the plugin. Just a guess, but it seems like iTunes is appending the ID3 tags or something upon import, and by design whenever iTunes changes the tag, it updates the modified date.
I did a few tests to see under what circumstances iTunes would change the modified date and it seems to only be if the tag is updated (not when the file is added to iTunes). If I drop an mp3 into iTunes, it doesn't change the modified date, just the date added, which is expected. Also, like I said, when using the older version of the plugin, iTunes isn't changing the date when files are first imported.
I think the options you came with up for the modified date are great.
For the 'Date Added' stamp, I think this would be good...
- Date follows MM's "Date Added"
- Date is set to when it was first synced with iTunes (the way it is now)
Thanks for looking into this.