If you want to go to the car for any reason, can you go back in?

Yes

Yup, hang on to your tickets

Absolutely

Pretty sure its kind of like a hopper pass, where you can go in and out as you please

Yes, absolutely! We left the park a couple times and were able to get back in by presenting our park passes at the front gate when we were ready. There isn't a short cut to re-enter the park so you'll have to go through the main entrance again.