Work in Progress: A muslin for Simplicity 6580

I cut out and assembled the pieces of the muslin for my Simplicity 6580 jacket.

Well, let's just say that it needs quite a few fitting adjustments, unfortunately I'm not at all sure how to proceed.

Let's start with the pattern. It came in two sizes: 7JP and 9JP. I'm not really familiar with these sizes. Is it the equivalent of a Burda "petite" size ? Not sure.
The chart on the envelope allows you to compare the standard measurements to the "Misses" sizes. the 9JP is closest to the Misses size 10 (which is closest to my measurements), when it comes to standard measurements anyway, so that's what I decided to cut.

You'll notice that while waist measurement is the same for 9JP and 10, bust measurment is 1/2" bigger in the "junior Petite" size, while hip measurement in 1/2" bigger in Misses size. The back neck-to-waist size is considerably shorter in the petite size, which makes sense...

Before I cut the muslin fabric, I added 1" to the total length, as well as 1/2" to the shoulder length, as, based on my own measurements, it seamed like the pattern would be a bit tight. I also added a center seam in the back, instead of cutting the piece on the fold, to facilitate additional fitting adjustments.

So once the pieces were sewn together, this is what it looks like:

Yuk, not quite the nice fitted look of the envelope. It's too big, obviously, and still a bit short I think. The added shoulder length was a mistake, but it's easy to cut back out. I'll probably have to make a narrow back adjustment, and maybe take in on the sides while reducing the back darts.

The sleeve is obviously too short (which is often my case, with my freakishly long arms...), but thanks to the elbow mark and shortening/lenghtening lines on the pattern, I should be able to fix that. I might reduce the curve of the shoulder top, it is very puffy as it is (it doesn't really show on the photos), but this may be due to the shoulder seam being too long and the sleeve cap sitting too low...

As for the front, I'm not sure what to do. There are no apex mark to help figure out how to improve the bust fitting, and since I'm not dealing with darts or princess seams, I'm kind of lost. Should I diminish the gathers and take in on the sides ? If I do, won't that move the neck yoke seam to the sides to much (over the apex)? The neckline seams a bit too wide as well, plus you'll notice my uneven shoulders (my right shoulder slops more than my left) which I might just adjust with the proper shoulder-padding.

In fact this has me wondering if I should have gone with the smaller size, while just adding more overall length (1" isn't enough I think) and adjusting the sleeves...

So far I haven't had a project with that much adjustment needed, I have my trusty "Fit for Real People", and I also got "the Perfect Fit" from the library, so these should help me figure some of it out...But I'll also take any advice anyone might have !