At the point when the two nuns are performing the transubstantiation after the barrels of wine spill out, this wouldn't have worked. According to the Catholic Church, only a priest can perform the transubstantiation of changing the bread and wine into the body and blood of Jesus. So this wine would've just been wine.
According to Father Ridley, SOME demons were once angels. Still, according to Christian doctrine/theology, ALL demons were once angels before being cast out of Heaven for siding with Lucifer and his rebellion.
The painting of the nun is the same Patrick Wilson painted in The Conjuring 2 (2016), which is set in1977, years after the events in this movie.