There is the proper way to bend metal, where it retains its strength, and the improper way. You can pop dents and dings without worrying about structural integrity. I do not know if it is possible to straighten a frame without weakening it further. I believe the proper way to restore a salvage car is to replace the damaged components. Some bolt on and off, but others are welded.
Someone welded on new chassis arms on both sides of this Focus, which seems like the proper method, but they also used body filler on the chassis!
I do not know if the body filler alone is responsible. Was it welded wrong?