I finally got back to this and made what I think is progress.
I followed your advice and put some tension on the quarter using a cheap clamp and a come-a-long hooked to the hitch receiver on my truck. Put a good load on it which made me nervous about the heavy clamp letting go and taking out my truck but didn't happen!
First I worked to push the sharp line along the bottom edge pushed out. I pushed hard from the inside along the pink line and slapped the high areas and folds near the orange marks:
Then I noticed this stiffening rib in the trunk was slightly buckled so I straightened it out:
