Lounging about with bf, who sews. Care for a man's take?
His first concern is the fabric used, says if you're trying this at home, it'll be "satisfyingly simple" in high-quality 100% wool but "an exercise in frustration" in the blend they use.
The three-piece sleeves *can* provide even better fit than standard two-piece. In this jacket, they might be just a design element. Either way, dont let one extra seam scare you. You can still sew the sleeve lining as two pieces. For lining, use silk with a bit of Lycra, it's easier to work with than synthetics are and makes a world of difference in wear.
He's not sure what you mean by "level of seaming" and says the six-panel design (vs four, some jackets don't have the side/underarm pieces this one does) can give a better fit, if you fit it in the right places.
The triangle on the back looks like it's formed by darts, which might be a "slightly functional" design element.
bf's nudging me toward learning to sew, but I'm not sure I'd find it as easy as he makes it sound. I was drooling over coats and jackets in a line of patterns he showed me, but sewists here said not to start with that. Now he says there's a company that generates patterns according to your measurements and I might like some of their designs, so I'll look at that once I've signed off here.
bf said more about your jacket, but I couldnt catch it all, thumb typing this on phone.
(Who was it who posted about long jackets 3 weeks ago? Bet she'd dig this one.)