This is a hard one. It does sound like the teacher has lost the perspective of being neutral with your son and perceives he's constantly misbehaving. Which will have the affect of making him constantly misbehave.
Can you arrange a meeting with the new principal - an informal "get to know you" kind of thing where you discuss your son's specific needs? No teacher bashing, just an introduction of you, and a discussion of your sons history in the school and ask for ideas to help? The teacher could certainly be invited.
I'm not sure I would change teachers at this point. That's a pretty drastic step, and schools don't like to do it because it sends the message, "that teacher was unsuccessful teaching the child, but this one will do better". Also it's a pretty public statement that your child is difficult to handle.
In our elementary school, they will not change a child's classroom unless the situation is extreme (two kids getting into fistfights, things like that). So you might be out of luck with that plan, unless you change schools.
I am not saying that some teachers don't get along with some kids. But something to keep in mind is that parents never believe their child is behaving in a problematic way, and teachers are trained not to let their personal feelings get in the way when providing education for children (again, not saying it doesn't happen). I am sorry you can't volunteer in the classroom more, so you can see for yourself what is going on (and actually, your presence might affect things for the better, as your son might not want to act up in front of you if he is indeed doing what the teacher says he is doing).
Going to the principal and accusing the teacher of favoritism or saying the teacher is responding disproportionately to "treats" (I don't exactly understand what you mean by this, do you mean the teacher was bribed by parents giving her treats? That sounds very odd) -- anyway, making accusations about the teacher is just going to sound like all parents' accusations against teachers. (A friend of mine has a son who got permanently kicked off the schoolbus for fighting. She didn't believe it and had a meeting with the principal to demand he be reinstated. They had to show her a videotape of him fighting on the bus to make her believe it.) Whatever you say to the principal, be very, very careful not to sound like you are sticking your head in the sand regarding behaviors that might have in fact been done by your child.
If you are saying the treats are something the teacher gives to the kids, and that some parents are providing them, you might think about it again rather than instantly saying this is non-ethical and should not exist in a public school. Some teachers reward kids for good behavior in class. (My son's second-grade teacher would say, "Good work helping your neighbor, Jaden, go get yourself a fruit snack!" and the kid would go to the back of the room and help himself to a little fruit snack kept in a bin. This was a big motivator for the kids, and a nice way to reward them for extra efforts in citizenship and scholarship.) Our school district has no money to give to teachers as a budget for extras like individually-wrapped fruit snacks to reward the children with. When the classroom runs out of fruit snacks, the teacher puts out a call to the parents, and some parents show up with fruit snacks or animal crackers or some other minor-league treat. I do not think this is a question of ethics at all. The parents are providing a treat that any kid in the room can get if he behaves well.
Maybe what you are reacting to is that the parents who like and get along with the teacher are more likely to provide the treats than parents who don't like the teacher. This is a natural outcome of them being in tune with the teacher in general, not a bid for special treatment for their child. It might even be that the teacher sees the friendly parents as being on her side, and it does relax her to have that vote of confidence. I don't see you giving the teacher the benefit of any doubt and certainly no vote of confidence.
Unlike RR above, I would say that if you are absolutely certain this teacher is picking on your child and it is not done fairly and your son is not doing anything the teacher says he is doing, I would give serious consideration to moving him. If he fails in a different school to settle down, then you will know it is your son and not the teacher. If the school will give you a chance to change him to a different class, you could try it, but be sure to say to anyone who asks that you think he needs a different style in the classroom, not a different teacher (in other words, while any teacher would be insulted if you said she was the problem, no teacher would be insulted if you thought a more disciplined or less disciplined style would be more effective with your child) If it is too much trouble to change schools and you can't change classrooms, just have your son stick it out. I wouldn't leave a first-grader in a situation where he seems to have a bad connection with his teacher, but I would be less worried if it was just kindergarten.
I'm just wondering if you have had your child tested for ADHD or Tourette's?
I'm suggesting this only because of what you have described...starting with the touching other children thing. Many children with ADHD ( and in that spectrum) feel the need to touch.
If he does have this problem, and is diagnosed, the school will treat him much differently. Better. Or at least they should.
He's at the age where specialists will now look at his behaviour and watch for these conditions.