a Q&A - Marji

From Melissa - The vintage diagram looks like you cut the underlining without s.a. and then hand stitch it to the s.a. of the fashion fabric, is that right? And I know I don't underline the sleeves, but what about where my coat crosses over itself for the double breast? It seems a bit unnecessary there if the wool is two layers already.

Making any garment involves making a series of decisions. The points you've brought up indicate you've thought this through, and I agree with your conclusions. If I were making the coat you're making, and adding a lofty interlining, I'd be tempted to underline the sleeves with something like cotton flannel (well preshrunk), and only use the interfacing for the front pieces since they overlap, as you indicated.
Then I'd construct the shell, and then mount the interlining as indicated in the picture I posted, using catch stitches and stitching it to the seam allowances.
If it were me, I'd probably add the underlining to the shell After the sleeves were in, after the sleeve heads and shoulder pads were in, but before the lining went in. It'll just be easier to work with without that interlining attached.