Marshal - I am afraid I am not sure what a DST is, so not sure if this will help but you have my sympathies.
At the start of any kind of meeting involving my mother's care , usually best interests, I make sure someone explains what the purpose of the meeting is, who everyone is and what they aim to cover. This should happen anyway but if it doesn't I ask for it. Make it clear if you have POA.
It may help if, in advance you write down all the important points you want to make as bullet points. It is very hard to look at emotional and sensitive issues like this, but it may help you get your views across and avoid you forgetting something with the stress of the situation.
Ask if you can record the meeting - they may or may not allow this.
Is there an ageing or disability voluntary group locally who could either advise you or possibly accompany you as an advocate? It may be that only certain people can attend the meeting, but they may be able to offer advice beforehand.
If something is raised in the meeting that you are not aware of, need time to think about or don't agree with, you should be able to think about it rather than having to answer on the spot.
I try, though it is very hard, to politely refute anything said that I don't agree with. CH can be very defensive, as you probably know but if you have doctors notes etc then these can help to provide a factual record of what has happened - instead of what someone wants others to think has happened. Very scary.
I hope it goes as well as it can. Please don't be too hard on yourself, this disease is a nightmare, even without the failings of the care system.