Thanks 
You can hide the quad in game with the “Actor Hidden In Game” option in the Rendering tab of the object.
I use a intensity of 6 for the sun and 16 for the spots, depends of your auto exposure. For exemple I used 1.0 for Min and Brightness (so no auto exposure) and 0.5 for exposure bias.
No tweak on the lights.
In the world settings I just set the number of bounces to 100 () and slighty decrease the indirect lighting smoothness (0.8).
I have not even try with a spot directly at the window, with some tweaks maybe it works … the reflector thing seemed logical to me 