I can see why you removed her door.....
Theres a lot of people replying not to worry about the vaping but the nicotine in vapes is known to damage long term mental health as whilst it might seem to "relax", its just temporary and creates a longer term cycle of nicotine addiction, depression and anxiety.
https://www.ncsct.co.uk/publications/young-people-resources-vaping
If those vapes hidden in the socks arent hers, then whose are they. Really, she may be very unwell but she has broken your trust (vaping in front of siblings, hiding vapes etc, going over her SM time) and if her friends are buying her vapes, (its against the law to sell to under 18s), then do they not understand they are actively choosing to worsen her mental health? , and doing the equivalent of giving sweets to a diabetic! . I think the advice from the pp with child under camhs is very helpful, including around simply telling her to leave the door open when she is in her room. This is part of safeguarding within the home, when she is more in control of her actions, the rules can be relaxed.
It sounds like her cousin is sensible and did the right thing in telling you about your dd's plan to unalive. Could you have a chat with cousin about her and explain you need the help of her and her friends to keep your daughter safe. Keeping her off SM and away from vapes etc.... The cousin could then explain to friends....?
I wondered if there was any activity or sport that you and your daughter could do together, maybe exercise based, even just walking outside, , that might help teach her how to lift her mood /relax her in other ways? Or even her doing something new like a painting class might help how she is feeling. Mind website has lots of info on self harm in teens and social media.
I also agree that you could not ignore her post about the teacher. She does need to learn this was a crime that could have affected her life forever, showing up on a dbs check every time she tried to get a job. Although you might not be able to get her to understand this right now...
These remarks about unaliving when shes caught doing something wrong. I think its good you are aware of these and when they occur so they dont become a manipulation. You cant argue with her to feed the drama or react, best to just keep calm, gray rock and carry on with whatever youre doing, holding your boundary. I think this is an incredibly difficult time for you and all your family, her siblings must be scared too, and you sound like a great, very thoughtful, mum.