As above she would only need to be here a limited time to qualify for ILR if she is appying with you. An existing visa is automatically cancelled when a new one is granted so there is nothing to do regarding that, just apply for a new one when you need to and before she is 18, assuming she is not going to qualify for ILR.

As I understand they are not bound by the 180 day rule. There may be some lesser restriction, I think it is related to the length of the period prior to obtaining ILR.