I was able to move both the light and construct the arches in two nights. It would have been much easier incorporating the arches into the original design, however, I found it was not all that horrible building off site.
Laying everything out on a 1/4 OSB board, I drew a 70 5/8” circle. Allowing for the 1/4 drywall, I cut and assembled the two sides. After that, it was glue, screw, block, glue, screw, and finish.
I find using the 3M #61 spray to hold the molding is a lot easier when doing things by yourself. Additionally, because I blocked it so well (wish I would have taken pictures of everything), I only used 1 strip of the 1/4” gypsum. I used two layers on the other partial arches, however, the ribs were made of 3/4” ply.
You can see the completed arch with a skim coat of mud, and the light moved to the “proper” location. Now I get to finish the drywalling – again!
Wait. I spoke too soon. I will be installing the niche next. More to come.
-- No matter how many times I measure, I always forget the dimensions before I cut.