Yes, commercial music releases tend to have a UPC. For many years now, the download and stream releases have tended to be assigned unique UPCs which are not shared with a physical counterpart.
Yes. All 16-bit lossy releases should share a UPC if they were distributed by the same distributor. All 24-bit hi-res releases should share a UPC if they were distributed by the same distributor. “Mastered for iTunes” releases tend to have a unique UPC because it’s a different master, although there is a history of labels like Warner Bros. Records sharing a UPC between the 24-bit and “Mastered for iTunes” releases. (And in this case, the 16-bit non-iTunes version of the release has a different UPC.)
There’s plenty of examples of this in MusicBrainz already, for example
Log in - MusicBrainz (Universal Music)
Log in - MusicBrainz (Warner Music)
Log in - MusicBrainz (Sony Music Labels (Japanese))
Log in - MusicBrainz (Sony Music (not Japanese))
No.