Ok, I got the promised follow up to my second call.
As noted, after that second call, suddenly data was no longer being charged to my account when I streamed my own, stored music from Google Play. Somebody, I think, fixed something!
--The tech who followed up said it was his understanding that all music from the Google Play app, including stored music owned by the user, was included in the music freedom plan. He said he still hadn't gotten official clarification on that, but it sure seemed that way and the fix to my own issue seems consistent with that understanding.
--He said several others experienced what I experienced. It may well be that TMO did something wrong in counting stored music and that they have now fixed it when some people like me complained.
--He said he'd follow up if there was an issue.
--Finally, I do note even now that after using Google Play to stream, say, 160megs, that a meg or two--not much more--is lodged against my data. That's minor, but he said the Music Freedom only includes actual streaming of music. If G=play is downloading playlists, photos, etc, that's going to count against data. My own experience so far is that it is not much of a charge--just a meg or two every time I do some major streaming. I can live with that.