Is your Mom trying to accomidate everyone or herself?
Parents forget that Kids schedules are VERY important and with some kids can take a week to correct 1 day of breaking that schedule.
I would do like Lynn M said. I would go for the time I had available, this would stick it to those who think you are trying to be difficult and see you are trying to compramise. and then after that I would ignore their calls, screen them. I have the same issue with my in laws; My family lives out of state, and my inlaws live 10 min. away. I have a child with major food allergies that they cannot/will not accomidate so I want everyone to come to my house where I can controll the allergin. They tell everyone that I am keeping the kids from them an that they "need to make an appointment" to come and see them. Well dropp in's are not wise seeing the kids are busy and we might not be home. They also tell others I favor my family because when they come in I will not cancel my families time for them. Selfcented people only see themselves and what betters them and that is all they will see. You need to see that that is what is going on or did she make a choice on a time that met the majority's needs or is she being self centered.
Keep to your time schedule.