I think (I hope) I finally have a solution to my shirt problem. It involved buying another yard of fabric and completely recutting the front, but I've now converted it to princess seams. I'm going to have to fiddle with the neckline a bit, I think, but basically I just pulled out a pattern I had for a princess-seamed dress bodice that I knew fit, along with the front, and traced it out to combine the two so that the armholes and shoulder lengths would still match up. (Look at me, I'm sort of patternmaking! :D ) So now I can get my tapering at the waist without having difficulty with the properties of the fabric. I may need to tweak the bottom of the seams just a bit, but I don't mind if it's a little loose. I just didn't want that pregnant belly look.
So hoping I can get the front and backs sewn together tomorrow. Maybe the sleeves if there's time. I'm going to save the neckline for after that, so I can tweak it to however it's hanging then.
Special thanks to mom2fur for her comments on this project so far-- really helped me with thinking through this!