Yes! I was just at the park today and dogs are welcome, on leash, of course.
I'm not sure either. To be safe, I wouldn't plan on taking your dog. If you do and leave him in the car, there isn't a lot of shade to park in.
I really don't know, but MT is know to be a love dogs state. Right now one could easily keep the dog in the car as it is cool, then romp about in the hills. Be aware, grumpy bears are waking up, just honk the horn 1st. (would have called them, will do so on Monday)
Get quick answers from Bannack State Park staff and past visitors.