You'll have to rip out the armhole stitching, unfortunately.
What I'd do is not close the armholes at all, and stitch everything else completely closed. Then you can turn the whole garment right-side out through one of the armholes. She has white trim around the armholes, so you can finish them off with white bias tape encompassing both layers.
FF9 cosplay? Hit me up!
I now have a Tumblr
. Sometimes I use it.