I’m sorry to read about your uphill struggle, you’ve been through enough.
I don’t know if this will be of any help to you, but perhaps look online at the site The Coroners’ Society of England and Wales. It has telephone numbers and departments to contact for queries and of course complaints.
My own dealings with the Coroner here in Yorkshire two years ago, after my husband’s unexpected and sudden death, seemed straightforward compared to the painful ordeal you are experiencing.
Thinking of you today and hoping you will find some peace soon.