I haven't made the pattern you mention however I have made many a garment out of cloth. All dressmaker patterns call for basting in the sleeve by setting in the sleeve side seam and bottom seam of sleeve meeting and than easing the sleeve in. Sometimes this will require more fabric for the back, which is why they ask you to ease and baste before you do the permanent stitching.
Sorry this has happened to you
, but fixing it now will make you a far more comfortable and wearable jacket. And yes I would loosely bast with a contrasting wool first