For keeping your facing pieces on the inside, after you do a top stitch, tack the edge of the facing to any bodices seams. so you’d be doing a couple hand stitches sewing facing to shoulder seam and/or bodice side seam. You can also use something iron-on like stitch witchery and iron this between the layers and this should also hold them together.
If you place a facing on the original original armhole, you’ll see that not only does it fit perfectly, but looks it could have been cut out from that part of the bodice. So after you make changes to the bodice, lay corresponding facing on the new armhole (matching at the shoulder), place your tracing paper over both and then draw facing outline but just extend your outline to the edge of the new armhole. I did something similar in our bodice facing pattern tutorial. I hope this helps.