My boy Pogo was car reactive. Not every car set him off, but he definitely tried to chase many types: any sort of delivery truck, slow moving SUVs, sedans with poorly tuned engines. It was, honestly, a bit dangerous. He ran behind me and clothes lined me with his leash more than once.
I don't think the reactivity issue will go away if you ignore it. Chewing on seat belts might get better with age, if Misty is not allowed to rehearse the behavior. I never found that treats were very helpful in managing Pogo's reactions. At the point where he had started a meltdown, he was already too overwrought to be interested in food.
Although he didn't !ove car rides, Pogo never reacted as severely as your Misty. I think part of the reason was that roughly five days a week he rode in a car to do something fun. Much of his puppyhood was spent at our horse barn. He loved visiting the barn. So, make sure Misty gets plenty of rides to fun places. You might want to try a dog crate in the car again, but cover the crate with a bedsheets so Misty can't see out. Pogo's favorite vehicle was our pickup truck. He spent most of the ride lying on the floor of the back seat; he could only see out if he stood up.
Unfortunately my neighborhood has few sidewalks. During Pogo's worst periods I walked him very early in the morning or late at night to reduce our car interactions. I also found he handled cars better if I brought a toy along and redirected him to the toy.
It sounds like you are in a good path with your thoughts around counterconditioning. For more background you might also want to read about flooding and trigger stacking. It's easy to push too hard, too fast, and set yourself back. I think one of my mistakes, early on, was getting hooked up with a trainer who was a lot more experienced with police dog breeds than poodles (and who wasn't as experienced a trainer with anything as she liked to think.) She saw the issue as pure prey drive and didn't recognize the anxiety. Fortunately she was a positive methods trainer, so she did no damage, but I would have had a much easier time if I'd found someone with a better behavioral background.