Sweater Mistake: Steek or not?

I have a knitting emergency!
I converted a pattern for a cabled sweater from flat front/back to bottom-up round. It has short cap sleeves worked from increases to the body with a rib edging added last. I have discovered that I did not separate the front and back when I was supposed to put in the armholes, so instead of a rectangle with armholes on the sides and a neck hole, I have a tube with a neck hole and no slits on the sides. I don’t want to rip out the 5 inches I have knit since I was supposed to separate the pieces. Is there a way to steek with no extra stitches such that I would only lose one stitch on each side of the steek? The yarn is 100% baby alpaca. If I do steek, should I separate them now, or continue and have a long steek? Thank you!

Welcome to Knitting Help!
Steeking the entire armhole will work. You’ll probably loose 3 columns of sts at the steek, one where you cut and one on each side where you stitch. I wouldn’t try any fewer than that and you might stitch down another column on each side especially if this is a large gauge.
http://www.knitty.com/ISSUEspring03/FEATsteeks.html

I think steeking will still leave enough stitches for the armhole. It is a size 5 needle. As the steek will be around the sleeve further from the body, I will be picking up stitches along the steeked edge. How could I best keep the ribbed edging from flapping back? In projects I have steeked before, I found that the wider steek forced the ribbing to flap even more than a normal picked-up edge does.

It may help to pick up more sts than called for around the armhole. That takes some of the tension off the ribbing and it may keep it from flipping up. I’ve added this kind of short sleeve to a vest before and it behaves well. It seems to be something like picking up sts for buttonbands: too many and the band waves, too few and the band flips.