I'm sorry you're going through all this. Sounds upsetting and exhausting and confusing.
I think you need to take a "long view" here. You're son is still so young. It's really hard to know what's at the root of these behaviors. Gifted kids are so intense, it's hard to know what's what. Teasing out gifted from ADHD (for example) is really for a skilled psych.
I love the idea of your going to the classroom to observe. Montessori schools/classes/kids are used to people sitting in the room, so it shouldn't be a big idea. Just write down what you see.
I'd take every data point I could from this experience. Write it all down. Try to be as objective as possible. Write down what the teacher said, what you saw, what happens at home, etc.
Next year he'll be at a different school, and you'll have another environment to see your child in.
