I can't say that I had a lightening bolt moment, but I do at least have a starting point, which is progress in itself. Here's a photo of the original pattern piece, with my first alteration cutting line marked in red. I know it's hard to see here; however, if you open it in it's own window, you can see it quite clearly. My plan was to cut out a box around the armscye, slide it over 3/4", then redraw the side seam. This was also going to increase the width of the shoulder strap.
Here it is after I moved the armscye and redrew the side seam, waiting to be cut out. You can see I also extended the shoulder strap a ridiculous amount; I wanted enough length to just be able to pin it to fit at the next fitting. You can also see that I've redrawn the armscye. My daughter's shoulders are broad, but not as broad as her back is wide, so I removed 1/2" of what was added when I made the broad back alteration.
Here's a detail shot of the armscye and shoulder.
Here's my new pattern piece...
And a side by side comparison with the original.
Comments, opinions and advice are most definitely welcome!