Amazing results can sometimes be accomplished on damage like this.
The problem is it takes a lot of experience to have the skills to do so.
A couple tips if you decide to try...
Never use impact tools (hammer, rivet gun, etc.) if your goal is a flat surface. They stretch the metal and it will not be as flat as it could be. In instances were a compound curve is desired, they are helpful because stretching is needed but that is not what you are after here..
To repair bends like you have, you need to find ways to cause a reverse bend directly along the line where the bend exists. It sometimes requires cutting a shape from hardwood that matches the shape of the bend in a particular area. Add a radius to the edge that is similar to the bend radius of the crease (in areas where the crease is very light you need to be using something with a large radius. In areas where the crease is sharp, you use something with a much sharper radius), and slowly massage with finger pressure in the direction that will reverse the bend. Start with the worse area first, and keep expanding the work area as the area with similar bend radius grows. You can also use radiused tools like wooden spoons or custom formed wood blocks and slide them along the peak of a crease while applying pressure.
A stainless firewall is actually tougher to do this on than an aluminium skin because it is so soft and will easily bend beside the crease while you are removing it. Because of that you will not be able to remove it 100%, so I recommend that after doing the best you can, you hide it by removing the gloss of the firewall by applying a machine turned finish (Google it or search the forum). I have done it a few times just with a 2" diam scotchbrite pad and disk, hand held in an electric drill. Everyone will marvel at your custom firewall finish and probably never see the faint remnants of the crease.
