It looks to me as if the light source is directly over her head (and if anything, it is ever so slightly to the viewer's left).
I don't think that your methods are unconventional...in fact, I've done it myself
)
I know that Rubens often did a small oil sketch in full color and his helpers blew it up on a grid and "copied it" onto canvas...color and all. Near the completion of the painting, the Master would come in and add his signature brushstrokes. In Ruben's studio, he also had those helpers who specialized, i.e.,those who were skilled in painting such things as animals and other who specialized in plants. Some of those old works had many hands producing the final piece.
Ahhhhh.....I long for the good old days.....