Just a few comments. Filing a law suit is not exactly non-confrontational. Leaks can be hard to find and of course first thing is making sure there are no technical service bulletins on it. Then at this point I think I would try to find a body shop that is willing to spend some time with a hose looking for the leak. You can do this yourself with a water hose. Once the leak area is found, then a fix can be determined. Sun roofs, door seals and adjustments, grommets not in place, etc. are areas to look at.
I had a leak in my 09 Pontiac G6. Water was in the trunk and couldn’t figure it out. After taking the tail lights out and using a hose, I found a single 2" grommet that was not in place. Problem fixed.
If I were you, I’d take out the seat and the carpet in the affected area and start going over it with a hose or sprayer and see when water appears. That rust is not a significant issue and really nothing needs to be done with it. You may need to have the seat professionally cleaned or re-stuffed, but take this stuff out first to mitigate the damage. It’ll also make it easier to find the leak. Some of the G6’s had a problem with the sunroof draining into the back floor because of too short a drain hose, so these things do happen.