It sounds like a real ordeal you've been through with that car, I'm sorry to hear that.
As far as SST repair goes, it is likely outside of their control. As I mentioned before these units are all purchased from Getrag for these cars and are not handled in house, because of that the scope of repairs allowed by Getrag is very minimal. They have only recently started supplying us with two primary parts for service... the clutch basket and the valve body (mechatronic). Those are the only components (major ones of course, apart from seals etc) available for repairing these transmissions, so anything else internal requires complete replacement. It isn't something the dealership has any say over, as Getrag does not make the internal parts available to anyone on them.
Generally speaking we see a lot of shift fork failures, some gear related damaged, synchros etc. All of these require full unit replacement without exception unfortunately.
As far as any sort of legal recourse you have, it is not likely as all you can really do is document how many failures there have been, spanning beyond just Mitsubishi but Ford and VW/Audi, BMW as well. They all use variants of this transmission and all have moderate amounts of failures just like Mitsubishi.
At no point though is there going to be anything that is going to indicate a defect in design or anything like that which would justify a class action or anything along those lines. You would need them to actually have discontinued use and even more likely have offered some sort of recall out of the unit before there would be a leg to stand on legally speaking. The fact that the transmission is still actively in use just worsens your odds anyway.
I would say the best thing to do here would be to get the vehicle out of the shop anyway, unrepaired, and get it to another facility that isn't more or less out to get you like it sounds like these folks are. You can always opt for a used SST for usually in the 2500-3500 range which can ease the pain a little bit. For $25000 though you might as well just go buy another one without any problems. Pricing the repairs that high make it sound like they just don't want to fix the car.