I agree here, if this dog was really being aggressive, your dog would be injured. Boxers have quite a rough style of play, very mouthy, body slamming, jumping on dogs, etc. and your dog just wasn't up to it. But if you were concerned, it's up to you as the dog parent to look out for your pup and leave if you're seeing something you don't like.Sorry about what happened and the owner of the boxer should have stopped their dog if your dog was crying out in pain. But I'm curious as to why you stayed after that? To be able to say they stayed for an hour more means you did too. Basically if you feel your dog is unsafe someplace it's your job as their caretaker to leave to unsafe environment. It doesn't sound like the boxer actually attacked your dog or Butters WOULD be hurt, just playing too rough and not backing off when it was clear your dog wasn't having fun.
As for the boxer being all over you trying to get treats... I don't really have much sympathy for you there. Bringing treats to a dog park is a big no no in my book and if you do you can't be surprised when you attract other dogs who want those treats. That drove me crazy when I still went to the dog park and my dog would be all over people who brought treats instead of running around and playing. I could tell when Jubel was overly interested in people they had treats. I'd warn them NOT to give him any or he'd never leave them alone, ignore him long enough and he'll go away. If they gave him some anyways they were on their own dealing with Jubel's attempts to earn more treats because they chose not to listen. From what you said the park was pretty crowded as well making bringing in treats even more likely to cause issues. Most dogs will just want to get some treats but some might get aggressive around food and other dogs. Just all around bad to bring food into a dog park, most have rules against that as well.
And bringing treats to a dog park? Expect to get crowded and followed by a trail of noses.