So, it turns out that spangle embroidery is relatively quick to do — yay! I brought some with me on my trip to Paris, and managed to do the waistcoat front on the plane in a few hours. I’ve now finished that, along with the revers turnback on the waistcoat (which is in my sewing room somewhere — I hope!).
Side note to those who suggested the waistcoat could be “zone”/cutaway — I’ve never seen a cutaway on a waistcoat, only on the overrobe (under which the waistcoat/underbodice shows), so I really do think it’s just that the CF of the waistcoat is cut on an angle.
Also, I mocked up the jacket (based on the waistcoat pattern) which fit like a dream. I’ve now patterned and marked the jacket pieces, and am starting that embroidery: