Been there, done it, bought the t shirt.
The decision maker is the hospital sw if you do not PoA, that person has to power to overrule the family, make a noise and they can appoint IMCAs to decide Best Interests, IMCAs are normally appointed when the person has no-one to speak for them eg family, they were appointed in mum's case as they were drugging mum against our wishes on one occasion. It was a traumatic meeting but we won and got mum out of Hell Hole Hospital.
The last time mum was in the Hell Hole, a consultant tried to frighten me into signing a DNR, I felt nervouse and would not sign as it seemed to be more like a pitch for time share than deciding on life or death of a human being. Mum got injured mysteriously, the hospital told us that "we can do what we like", I was alrmed at things I saw happen both to mum and other patients. A broken arm left unsupported and obviously broken for 24 hours, only taken down to A&E because I made a fuss. The hospital seemed uninterested, I reported them for elder abuse.
Thankfully, the team carrying out the repair to her arm were decent & professional human beings who discussed everything with us. They noted the absence of the DNR form, I told them of my concerns, they accepted that we wished for the best to be done for mum, if she had profound complications then they were to take what they felt was best at the time. As it happened, they decided an operation was too dangerous and just reset the arm, we were told she might lose the use of the arm, thankfully they were wrong.
Before going back to the hospital, aquaint yourself with the Mental Capacity Act, do what you feel is right for the person, don't automatically believe what the hospital tells you - remember where the hip was broken.
Just keep using the term 'best interests', it has a standing in law.
ps If you feel the need to make a complaint about the Decision Maker and that person is a SW, they are mostl likely employed by the county council not nhs.