What more do you want? The mechanics taken out back and whipped? Cummins said they would give you your money back. That would restore you to before you met them.
If you truly believe they tried to rip you off, rather than just being incompetent, you should file a complaint with the governing authority.
As a caretaker of a special needs person, I would advise you NOT to play that card. It has no real bearing on the issue other than make you look bad. Just state that for undisclosed reasons, you want/need a dependable, operating generator, although some could say "Go to a RV park and use their electric."
Take a deep breath and good luck in your quest.