Retrofitting the coat pattern - next time, make the muslin first!

In response to my plea for help, several of you responded (thanks!!) to reassure me that gussets are not really all that mysterious and that I could do it.  I am not all that afraid of the sewing of the gusset - what has required more mental effort is the notion of exactly where the gusset should go, and what shape and size it should be to make this coat actually wearable.  This problem brought the sewing on the actual coat to a crashing halt.  I backed up to muslin stage.  If only I had started with such a step, instead of having to down tools in mid-project to "fix" a bad pattern!  (Will I ever trust Burda again????)

I assembled a muslin out of a rather stiff canvas-y fabric so that I could test my gusset-making skills.  I've taken photos along the way to document my attempts, failures and "progress".  It does not really feel like progress when you start to progress from 3 steps back!

Here are my muslin demo pictures:

On the right sleeve (to the left in the photo) you can see the original design.  I have no range of motion because the arm attaches to the body of the jacket far too low.  When I try to lift my arm, the body of the jacket comes right along.

The next step was to figure out how far into the jacket body to slash so that I could insert a gusset.

I did this very unscientifically!  Wearing the muslin and looking in the mirror, I took a white marking pencil and located the top of the slash by feel/instinct where it seemed that if opened up to that point, the "hinge" connecting the sleeve to jacket body would give me enough room to move.  Then I had my husband figure out where the corresponding point would be on my back.  
This turned out fine, somewhat to my surprise.

On the left sleeve (right of the photo) I have inserted a gusset which turned out far too big.  I did not modify the original underarm seam.  With that very low underarm, I needed the gusset to be this size to achieve an acceptable range of motion for my arm.  However, it is UGLY!

On to my next experiment.  I realized I could turn the original upside-down U shaped underarm seam into a higher pointy-cornered seam.  In the next photo you can see my pencilled changes on the pattern piece for the back, including the line up into the body of the piece, which marks the slash where the gusset will be inserted. The effect of this change is to raise the insertion point for the gusset, meaning the gusset will not have to be quite so wide to give the same range of motion.

And here you can see the result of the smaller gusset (inserted in the right sleeve, to the left of the photo).  It's a little cleaner looking than the big gusset on the left sleeve.

I look a bit happier with this experiment now.

Wish me luck translating this change, and a few others I thought of while puzzling over this problem, into my actual coat.