[Shiny]

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:

[Shiny]

Then the good part - This indentation would oil can.. which was my biggest motivator for pulling on the quarter:



The concave region right above the body line was pushed out some. After I straightened the rigid fold, I knocked this concave region (orange) in while pushing out along the convex upper section (red) and it held!



With much amazement, I found the buckled section that oil-canned had straightened out and was actually holding some shape!!

This is where I ended today. I plan to weld the tear next, then strip and start looking more objectively at the shape... but so far, it's better than where it started.