If the bluestones were also previously another circle or structure then they will have been transported from a third location (other than the originating quarry).

Indeed, and that could explain the fact that they came from different locations in Preseli, not a single quarry. It seems they occur in convenient blocks and suitable ones could have been searched for in the general vicinity and assembled into a circle, only to be subsequently moved as a whole to Wilts. That has a certain logic to it, whereas Stonehenge people going there and picking stones from different places when they got there is much more puzzling.