I had some more trial/error work on this. no solution other than using another name instead of “Id” 
I have opened a new post with a bit more description of the problem and a simpler/generic code sample here: "Facet" fails to map "_id" field of a projection to "Id" of class (BUG?). You may set a “watching” or “tracking” notification level on it. I have also a link here not to lose the track of it 