I think there's probably a fair bit of missing context here for me as a stranger to know what's going on, what's been misunderstood, and all sorts.
The safeguarding concern of the daughter in the bathroom - the HOY thought she'd gone back to class, the class teacher thought she was with the HOY, the daughter went to the toilets rather than straight back to class. I can read that she was crying and had a panic attack. That she was crying when she left the office? Like full on weeping or fizzy nose crying? Is this the same story you have from your daughter and from the school?
The tiktok thing - none of us know, you need to ask the HOY exactly what she means by a safeguarding concern with the tiktok account because there's a whole lot of guessing going on. If the account you know of is what she's referring to, why is there a concern, is it the content or is it that it exists? If they want you to be in a position to do anything about it, it's really unclear what they expect.
Why was she crying in class before getting sent to the HOY? What was the discussion with the HOY? Was it a frustrated cry? Overwhelmed? Upset by something? I don't need to know these answers but it's all context around the why and what of the situation and what should be expected of the teaching and pastoral staff.
Does she have an IDP? Or whatever the equivalent in England is? What is the support they've said they will have in place and is that being given? It's sounds like that's a no and that's where the area of concern is, not the potentially snarky attitude of the HOY. That's a separate issue.
Is the school in general shit with supporting neurodivergent pupils or young people with mental health issues? Some are so much better than others. What's the nuture offer? Do they have an educational psychologist? Councilling?
There are just so many questions.