OP, I’m approaching this from the perspective of a teacher, as I’ve dealt with students like your DC before.
I think that you should contact the school, either by email or request a meeting.
When you contact them, the worst thing you can do it appear combative. Schools are drowning at the moment, and if they sense that you are going to be difficult then they just won’t listen to you. You need to approach the issue as one of your DC needing more support. Explain that she’s on the waiting list for diagnosis, and request a meeting with senco. It would be good to have some solutions in mind to suggest for meal times, bearing in mind that students must be supervised at all times.
Respectfully, eating in the toilet cannot be allowed at school, so while the teacher on duty was perhaps too harsh with the way they approached it, a line needs to be drawn somewhere. However, if your DC can’t handle eating in the playground due to noise, a solution needs to be found. Loops, or noise cancelling headphones could be a start. Longer term, a quieter place should be made available, although then it becomes a supervision and funding issue. Again, suggest solutions if you know of anything that could work.
Remember that the school is there to work with you, and not against you. If you approach it in this way then the conversation and outcomes will be so much more productive.
I hope it goes well for you and your DC.