At the risk of just confusing you, the way I do dormers is to create the front and side walls of the dormer but leave them sticking out of the roof, extrude a short piece of the dormer roof (roof by extrusion) then use the Join/Unjoin Roof tool (outlined in red on the image attached) to extend the dormer roof to the, in your case, mansard roof.
The side and front walls where they intersect the roof plane will allow the intersection line to be picked and trim to complete the lines created by the Dormer Opening tool.
Once the cutout is created you can trim the side and front dormer walls to the dormer roof and then trim the base of those walls to the underside of the mansard roof.
Then copy that dormer along the mansard, making sure to include the opening cut.

The circular chain of references error - not sure but you might be able to lose that by disassociating an extrusion from the workplane.

I hope that is some help but others may have better or clearer suggestions.

I use an in-place component void and cut the roof as necessary.