OP, you asked me what I think, as a teacher.
I think that misbehaviour at school should be dealt with during school time time by the school, especially with a young child, such as your ds. Yes, you need to support the school, and not undermine or downplay what they are telling you, but if he kicks off during the morning, they need to deal with it, effectively and swiftly, a delayed 'punishment' at home, for something he did under their control is wrong. He will struggle, at that young age to understand why he's being punished for something he did earlier. I would stop all this teacher calling you and telling you to stop his tv time business right now. She's passing the buck rather than taking control and sorting behaviour herself.
Is the teacher inexperienced? It does sound as if things are allowed to escalate, whereas if they were nipped in the bud, there wouldn't be such an issue. The mat thing at story time is just poor class management and a bit daft: "Johnny, you're sitting here please, next to me and if you get up and move then I'll ask you to sit on a chair over there.'
I would suggest teacher gives your ds a notebook which she writes positive and negatives in, for him to give to you every day after school. You sign it, comment if need be and return. This will show him that you and school are a team. It will give you a record. Then make a 'reward' chart and let him have a sticker for every positive comment. After x amount of stickers he could have a treat.
You could ask to see the head and request a behaviour management plan. Ask for clear indications regarding their procedures for dealing with this type of behaviour.
Or, if my hunch is right, wait until year 2, which isn't long and see if things are much improved due to a different more experienced teacher.