About your background, have you considered practicing thumbnail sketch abstract, quick, on the side and see for yourself? this could be interesting. I like the present background as is, it does something special for this portrait on that side of the face, adding presence.
You are dedicated to continue to do your best and a great job on this and it shows! Your prep work and beginning approach are good and I like the direction that you are going into. Don't worry about the mess, just keep painting. So far you are not confused about it, that's good and want to avoid that with a sense of resolve as you are. Your drawing and composition look good. I also like the direction of the strong diagonal that leads toward upper right and out of the picture, works real well here, the figures keep me in the moment not wanting to leave but stay and enjoy, so it's quite welcoming. The beautiful story that this picture tells, is still there, intact!
This is what I have also and works great for me! I don't miss painting and drawing in the dark at all! 6 bulbs, 8' feet high @ 35 degree angle behind me, 100 watts,1500 Lumens, 5000 K. What a tremendous difference, thanks so much for sharing! I also have to be much more aware of what clothes I am wearing in the studio to avoid weird and difficult color reflections bouncing into my painting and reference photo and painting palette and still life. Fantastic!
I like your suggestion of purple for the background, and have you considered a muted purple or a light muted green. I also recommend that you practice quick color thumbnail sketches using simple abstract shapes on the side and explore and experiment before committing to anything, because we really don't know what's going to work right now. Nice photo to work from, nice pose and good composition. Sargent never left anything to chance.
He did leave warning signs, quite a shock to also take away "everything" he brought with him. However much of what he brought can be learned by anyone if they wish. Much is referenced in "Andrew Loomis" books and his color charts. I will still miss him regardless, great to work with him looked forward to it, good hard work though. I learned a lot from him, now for me to continue to incorporate it and develop through my own paintings, my own decision.