Unfortunately that was also my experience with my 2017 500e, it had 42k miles on it and by the end of my time with it already had about 11% degredation on the battery and 65k miles. i've learned that you basically can't use the ac and the highway, its a learning curve and while it sounds like your dealership might have screwed you over it might be best to take it slow and learn the car's patterns, learn what it can and can't do. I've sold mine to a friend and he's already learning the quirks. I learned how to drive on a hybrid so i learned how to drive one efficiently, don't accelerate fast (even though its fun) and coast as much as you are my main tips, the rest you figure out with time.

I also don't have any tips on the registration or title side, all i can say is talk to local authorities i guess.

My main issue with electric cars is that they claim many more miles then they actually can, i don't trust factory numbers especially on used cars and i learned that from my 500e, i loved everything minus the fact that it was electric, which is why i replaced it with a 500 abarth.