I don’t think that a digital release is only the data that you can download. I think that a digital release consists more or less of the data that has been provided to a digital store. UPC and ISRC codes have been provided to iTunes by the music providers/submitters (iTunes FAQ).
From my viewpoint, iTunes barcodes are part of a release even though they are not included in the downloadable files. iTunes is not letting us download all the data that was provided to them so in that sense they are stripping some information from the end product.
I’m not sure what you mean by this. iTunes barcodes are provided by the submitters so the barcodes are always assigned to the releases. Even if the submitters use the same barcodes on digital releases as on their CDs, that’s their decision to make.
You’re right that it’s good to keep in mind that these kind of questionable sources can’t be trusted alone. In those cases we can and should verify the barcodes with the UPC lookup to be sure. (Relevant comment from an old thread.)