I have a slightly different response to everyone's suggestions.
I wouldn't want to be in a group which was so in thrall to this woman. I was once in a group like this, and it's absolutely Queen Bee syndrome, and even though I liked some of the rest of the group, the fact that they couldn't see it for what it was, and just limply put up with her shenanigans, put me off them as friends.
My good friends know who I am and how I behave and would not allow someone to be mean about me or to me like this.
I would remove myself from the group on the basis that I don't need to go walking to be dissed by this woman and have everyone go for drinks with her, I'd rather go home and sit with my cat than endure that nonsense.
You sound like such a mature, fab and empathic person, OP, you have been a good friend to her during her loss, and she's obviously got issues if she's dumping you on meetups for 'old friends' when you are also a good old friend. I wouldn't accept being cancelled once you had already set off. The fact that she thought she could just say 'I'll see you several hours later, as I met someone better' is so insulting. I wouldn't have even included her in a nice WhatsApp the next day.
I wouldn't bother speaking to her, catching her eye, pleading with her to behave better; clearly, she's getting something out of causing this rift, and she's going to carry on. Even if she had a problem with you, she could just quietly remove herself from the group, or speak with you privately- she likes the whole drama and is going for drinks afterwards to bitch about you, otherwise she would tactfully go home!
Read 'Let Them', the book, and contemplate how to let her behave how she wants (rudely, not gratefully) and then let you think about what behaviour you need from your own friends, the fact they didn't have my back and just wanted to pander to her would make me think they are nice, but not ultimately your 'ride or die' friends, which you were for her in her hour of need but they are not for you. I can tell you are too nice, OP, which is why she's walking on you.
I'd arrange some one on ones with the friends you get on with best out of the group and accept the group in its current form is no longer functioning.