Although the gall description doesn't quite fit (this gall was too full),
perhaps Lasioptera solidaginis as per Gagne's RedBook, page 157
September 12, 2016: Lasioptera solidaginis as per R. Gagné (Thank you!)

September 5, 2015: collected gall on Goldenrod, Solidago canadensis
April 20, 2016, on/before: emerged, 1 larger wasp, 2 smaller wasps
April 30, 2016: cut open gall, full of life, including 1 larva, 2-3 fly pupae, sacks with communal wasps almost ready as adult wasps
Taped the gall back together as best I could
May 2, 2016: in container, more live small wasps, 1 dead gall fly (spent pupal case still partially in the gall but fell off)
May 4, 2016: larva was out of the gall, photographed and now in alcohol
May 6, 2016: adult emerged
I think the larva and the pupa are the same species as the fly.