I’m sorry for your loss and hope that this post may still be of use.
While I haven’t attended coroner’s court for a relative’s death, I was called as a witness after a friend’s death. It was slightly different in that the cause of death was pneumonia following serious traumatic brain injury but the process will be similar.
The coroner court aims to act as a fact-finding process to determine the events leading up to the death rather than attribution of cause/responsibility. This doesn’t mean that there won’t be barristers there (I had an attempt at cross-examination by a barrister representing the alleged driver of the vehicle that ran her over - but when you see a car coming at you at 50-60mph in a built-up area with a 30mph limit, you don’t remember much apart from running for your life) but I don’t think that barristers are considered to be an essential part of the process.
If your relative had a mental health advocate, they may be a suitable witness since they will be independent of the NHS trust(s) in question. Some mental health charities eg Mind or Rethink may be able to offer legal advice .
I hope that the inquest helps bring you and your cousin some answers, even if they can’t bring your loved one back.