Congrats, your muslin already looks better fitted than most of the shirts on earth:)
But here's my finicky opinions and what I'd do:
To address the balance issue, I'd remove the sleeves and open the side seams, then pinch out a tuck across the back as marked with the blue lines in back, first image. This will remove the bunching excess at the small of the back and, as you go further, will start to pivot the side seams backward, leveling out the crossgrain in front but probably requiring more circumference in front, indicated by the lower blue lines, same image. When you stand leaning back, the front winds up needing to be wider than the back if you want the side seams to be strictly vertical. The armholes will of course get smaller as a result, so just drop their bottom edges to get them back…but also see comments below about the wrinkled side seam.
These changes probably won't also fix what I see in the second image but the issues may look a little different afterwards.
It's clear your yoke IS a bit too long at each end, certainly by today's tighty-tighty standards, so I'd bring those ends in at least 0.5 inches; the armholes will come along, too, of course, which will make the sleeves tighter across the top of each arm. But that’s how it goes if you don't want relaxed yoke/sleeve-cap fit.
The back itself (and so the front, too) is too long on the left side in the second photo; I'd commit to having the pattern be asymmetrical, either taking out a wedge across each from centers to side seams (NOT a dart; I'd close the pattern to make the wedge go away), OR experiment with taking a deeper tuck on that side in the previous step…or maybe a bit of both?
The yoke is also too long going into the neck, especially on the more wrinkled side, better seen in the right-hand pix here:
I put these two pix together (the side-seam and neck) because in my view they're examples of the same problem: The garment is too long going into an obstructing body part (arm/neck), so it's bunching up around the body-part like water trying to stream around a tree trunk (we live on a river). Taking out some side-seam length as described above will help for the side seam, but in both situations, the most obvious thing to do is simply to cut the armhole lower and the neck wider, so the excess fabric bunching up is removed.
As I said, what I'd do; hope it's some use to you!
Edited by dpcoffin, 22 June 2017 - 01:32 PM.