Plan:
1. Copy existing jacket pattern to tissue paper. Will use tracing paper...
2. Build a jacket using some scrap stretchy fabric of some sort (not the fancy stuff, not exact stretch, but hopefully close), minus zipper!
3. Check fit.
4. Adjust fit, deconstruct adjusted jacket.
5. Transfer adjusted jacket to pattern
6. Build jacket with new fancy softshell.
Getting the collar length is going to be a bit of a challenge.
And I think the bottom of the jacket probably should be a straight line... I think I flipped the wedge part. I will have to watch that.... I guess I can cut the jacket slightly long and adjust the bottom, however, as it needs to fold back over on itself.
Someone's video on shortening a separating zipper.

Adjust the shape of the shoulder pieces so they lie flat, before proceeding. In the photo there are pulled waves in them.