I am very sorry to hear about your mother's death, Fizzielove - it is bad enough to lose a beloved parent in any circumstances but to lose someone like this can seem unbearable. You left someone who was ill, but still in the midst of life, one day - looking forward to seeing her imminent grandchild and not having said her final goodbyes to her family - and then returned soon afterwards to find her insensible, after a decision had been made to hasten and ease her death. The shock and grief of this must have been terrible. Personally, I feel that close family who have been visiting regularly, and obviously care deeply about the ill person, should at least be informed, if not consulted, when this final procedure is being put in place. The way this happened has left you at a loss, with no information to help you make sense of it, or come terms with it.
Your grief has been accentuated by the fact that you were about to bring a new life into the world and you wanted your new daughter's life to cross with your mother's, albeit fleetingly. I am very sorry that you did not get the chance for this to happen - it is a deep human instinct to see the baton of life passed from one generation to another and you were cruelly deprived of it. It is hardly surprising that you feel devastated - you are feeling just as a loving daughter would.
My first advice to you, Fiizzielove, is not to minimise your feelings - you need to grieve before you can possibly come to terms with this. Cry, and express your thoughts, fears and sadness to those around you. I would also make an appointment with the consultant under whose care your mother was in hospital. He will have access to all your mother's notes, including those made on the night the decision was made to begin the LCP. You should be able to find out exactly what the circumstances were that led to this procedure being decided upon and what part your mother played in it. I hope that learning what exactly happened will give you peace of mind on one level - when you know nothing about a situation, it is natural to fear the worst. Proper knowledge, I hope, will allow a natural process of grieving to begin - at the moment, it is being blocked by your feelings of helplessness, fear, and anger at not knowing exactly what happened.
Lastly, you are a new mother, with all the fatigue, uncertainty and disrupted hormones that go with it. Look after yourself and try to take comfort from your baby girl. One life has gone and another has come into the world. When your daughter is older, you will be able to talk to her about her Granny that she never met and show her photos. I'm sure you don't need me to suggest it, but have you thought of giving your little girl her Granny's name as a middle name? It is trite but true that love never dies and your mother will live on through you, your new daughter and her children after that.