I have been in this situation with my DH. You need to take a deep breath and try and stand back a bit and take an objective look at things. I know it is hard, but torturing yourself with guilt is not a good route.
My DH had a neurodegenerative illness and following a fall and surgery went downhill very fast. He was physically incapacitated and mentally in distress. The medics were clear that they had no further treatment to relieve his distress, in spite of their very best efforts.
One day he developed a temperature - presumably some infection - and they were all set to blue light him to hospital when I intervened. I asked them what they were dragging him back to life for? - what was his quality of life? I had PofA and instructed them not to take him into hospital for drips and tests and heaven knows what, but to make him comfortable and let nature do what it would.
The first thing I would say is that he was made very comfortable - they kept his mouth moist and his body clean and sore-free and administered a sedative at any sign of distress. And he had no fluids. He died very peacefully.
So I have had to live with actively making a choice for him to die on his behalf. Yes - it was hard, but I was very clear about his best interests. Yes, there was guilt, of course there was, but the alternative was unthinkable.
What I would say to you is that you must ask yourself what you would be saying to a friend who found herself in your situation. And what would you be thinking. Would you be thinking that your friend was doing their absolute best in a very bad situation? Would you be telling your friend she had absolutely nothing to feel guilty about?
You are doing your best in a deteriorating situation. When your mum was clear in her mind she was expressing a wish for her life to come to an end, and even had a plan to achieve that. Her wishes were clear. Now that she has become worse, there is no reason for you to think that this has changed. She is hearing the end of her life and whatever you do or do not do will not change that.
It is important that you allow yourself to know that no-one has a simple answer to all this and you are doing your best. Watching someone come to the end of their life is a challenging and profound experience that demands a lot of us and has no clear rule book. Every person is different. It is an experience that many of us have never been through and we are having to learn at a great speed.
It is clear from your posts that your expectations of what her dying process might be like are uncertain in your mind. I think you should write a list of questions and ask to speak to the doctors so that you can have as much clarity as possible, even though they cannot predict with total accuracy.
But please remember that there is no right or wrong thing for you to do here - you are feeling your way through a bit of a minefield that anyone would find hard.
I will be thinking of you and wishing you strength. Above all else do not feel guilty - you have noting to reproach yourself for. Take care.