All done
After all my fretting about how shrunken the sweater looked as I was knitting, and whether I had miscalculated the fit, everything relaxed in the wash as planned, and the fit is perfect, just the right amount of ease when buttoned. And the fabric drapes beautifully — so glad I sought out a longwool yarn. 
I think the back is my favorite part of the sweater — I wish the front had more of this look, but once I committed to reversible lapels, my options for the center panel were limited.
The collar can be buttoned up to the neck on chilly days.
Blocking
This washed beautifully. I’m doing minimalist blocking — no pins, just a laying it out and verifying that the measurements are still as expected. All those lumps and bumps I. The sleeves from multiple re-knittings disappeared like magic.
Button choices
with first button band on, it’s time to review the button choices. The larger mother of pearl and coconut shell buttons I thought I would like based on swatching look a bit large on the finished band, so the smaller smokey shell buttons look to be the best choice.
With my button choice finalized, it’s on to the second button band, and buttonholes.
Collar, revised
I ripped out the collar and neckband, picked up fewer stitches along the saddles and back neck, and reknit a narrower collar — narrower by almost the width of the button bands to be added.
I will have to give up on the option to wear the collar buttoned up all the way and loosely folded over, but I’m much more likely to wear the collar open, and it will look more balanced this way. Picking up fewer stitches also gives a closer fit around the neck and I the necline did not pucker as I had feared.