Ripping and testing and re-sewing are not my favorite sewing activities, but sometimes you just have to force yourself to do them. In fact, I rip and re-sew quite a lot because I'm a perfectionist. However the ripping done on this coat is far beyond what I'd normally expect to do in a given sewing project.
I basically wasted 2 sewing days ripping out what I had sewn, making changes and then advancing back to where I should have been last week. Now I have a functional gusset and a lining that shows more of the promise of the coat-to-be.
After I figured out, using the muslin, where the gusset needed to be and how big it should be, I transferred the necessary details to my pattern and then to the side fronts and backs of my partially-sewn lining. Of course, this meant I had to rip. While I was taking out the side/underarm seams, I decided that at the same time I would have a try at fixing another problem with the lining as first sewn - the sleeves felt too bulky and too narrow. So I also removed the Thinsulate from the sleeves.
I recently bought a life-time supply of white basting thread from a charming lady who runs a tailoring supply store in Montreal. I own needles. I have no excuses! I didn't even mind the hand-sewing activity. I laid the very bulky coat out on my cutting table, as you can see in the picture, and did the sewing while standing there. It was quite Zen... Changing the underlining to a less-bulky one, and paying attention to the stretch factor solved my too-narrow sleeve problem very nicely.
I sewed the new underarm seam up to but not over these lines of stitching. Then, I slashed up between the lines of stitching.