I think you're being quite horrible to a child, actually.
You've said that she's in the middle of an ADHD assessment, so clearly there are some suspicions she is neurodivergent.
People with ADHD really struggle with looking after themselves and their environment. They don't choose to have executive dysfunction and to find task initiation so difficult: it's how their brains are wired. You suspect she has ADHD and yet you've referred disparagingly twice to her "filth". You've called her room "vile" and complained about the lack of care. "We take the phone off her and she just sits there all day and doesn’t lift a finger." Well, yes. Taking the first step to doing anything can be hard. Struggling to get started is a symptom of ADHD.
Emotional dysregulation and rejection sensitivity dysphoria are also common. So, not being able to express her feelings properly and being terrified everyone in the club is going to hate her when her mum starts working there is completely understandable. People with ADHD live with a different baseline of anxiety. Constantly being a bit anxious is their normal.
Depending on what type of ADHD, she might find that some kind of exercise helps her manage her mood better - if you're a gymnastic coach, it's not really a surprise that she's also quite athletic, and that gymnastics is a good outlet for her.
If you've been out of the game a long time, I do understand that getting a job as a coach is going to be difficult to turn down, as you can't just get an alternative job somewhere else. I do. But I don't understand why you think your daughter should attempt to understand that when you haven't put one tiny jot of effort into understanding her.
You complain about "pandering to her" and "I can assure she wasn't brought up like this". You basically dislike her and think she is bringing shame on you for being wired in the way she is. Well, assuming she's yours genetically, you made her. She can't help her genetics.
"She has it easier than most people." Well, no. The world is not built for neurodivergent people, and she's in a minority.
Being a teen is difficult. Add ADHD into the mix, and it's even harder.
I have ADHD, and I feel really really sorry for your child.
If you genuinely think she has ADHD, you have to stop treating her like she's neurotypical. You have to put some effort into understanding her differences, not judging her for those, and helping her develop coping strategies.
She can't help but think this change is going to make everyone hate her. What you can do is acknowledge her feelings, understand why her gut instinct is to panic in that way, and then help her rationalise why this change isn't actually going to ruin her life. You can't have a productive conversation by starting from a point of dismissing her feelings.
She probably works really hard to come across "as normal" when at school. This makes it even harder to manage her feelings at home. The fact that she has such wild mood swings at home shows that actually, on some instinctive level, she trusts you hugely. I'm sure it's not easy with a teen who is always having extreme emotional outbursts but that's the job as a parent. You deal with the good and the bad, and sometimes, there's a lot of bad!
Rather than have a go at her for her room being a mess, give her 30 minutes to do something small. It doesn't have to be tidying the whole room. Find some really small and obviously easy task like stack all the plates and put them on the desk. Encourage her to take a 10-minute break playing on her phone. Then, give her another 30 minutes and another specific and small task.
If it was possible to get another coach job elsewhere, I'd take it. But's a very niche job, you don't have any recent experience of it, and they don't grow on trees. I don't think you should turn it down. But if you were more accepting of your daughter's challenges (living with ADHD), perhaps she would be more understanding of yours (reentering the workplace in relation to your specialism).
Just try to see things from her perspective. Then, with that different view, try things differently with her, and help her to see your perspective.