All of the above!
After her allegations I would never let that child in the house without her parent there again. I think you've posted about this child's behaviour before?
I would also be very very cautious of saying anything like "she accused me of abuse".. because a CF will take that script and run with it.. "She said my child accused her of abuse and now she's dumped us - (subtext - I've no idea why poor victim me, all I wanted was endless free childcare for my difficult child but its odd that she should mention abuse isn't it ")
It is pointless trying to honestly explain your reasons to this woman in order to be nice. She is a cf.. and her child is a cf. She hasn't listened to you, dismissed it every time you raise it.
Maybe try soft diplomacy as a first step - a firm fade out and see if that works out. No excuses because, speaking from hard learnt experience here, a CF will almost always work on a solution to any excuse you give.The best squasher is "Why?" , "because I just don't want to today."
I think you've tried hard to discuss it and she just railroads you. Situations like this build up over time until they become unsustainable, so don't blame yourself but you do have to prepare to be a lot firmer, even if it feels rude or blunt. She's actually quite blunt with you isn't she? And very good at shutting down conversations when she wants to.. so she's the one setting the tone.
Stay firm. She already knows a heck of a lot about your routine and schedule so this will be difficult, but you need to really limit the information about what your plans are from now on. She will push back, make you feel like you are being nasty or unkind... but YOU are NOT the person who is being nasty or unkind. Give yourself permission to say no.
You've mentioned you're concerned about how this will affect the friendship group, but I think that's because you are concerned about what CF might say about you if you say no to her. Actually that's a red flag that you feel a bit threatened by her. She's not a friend and she's not treating you like one. You need to practice the Gallic Shrug. Try to keep it very low key, don't discuss it unless its with someone you really trust.
I feel sorry for this child in a way because I don't think the mum does her DD any favours by not dealing with these issues. But I feel even more sorry for you, and even more than that for your poor DD, keep her booked up and busy with people who treat her well instead.
I'm sure you can do this, its uncomfortable, but its reached a point where its necessary. Just remind yourself about dangerous actions, taking your child's things and locking her out of her own house in freezing cold, every time you need to stiffen your resolve.