Steeking freaks me out too. I tried it once on the armholes of a doll sweater to learn the technique. It did work. I also used part of the sleeve to make an armhole facing. So the cut edges were enclosed.
How much too big is this jacket? You could make larger side & sleeve seams. But that might mess up the way the bust shaping falls on the body.
Sometimes, it is better to give the jacket to someone who can wear it. And reknit a new one in a smaller size. Of course, her friend could unravel the whole thing & start over.