As per the design, every time unleashed AP (irrespective of roles) is booted up, it searches for available master APs in the same network. If yes, then the AP would join the existing Master AP, else the AP will decide itself as a Master AP and wait for member APs to join.
In your scenario, Config-A would override the new AP config.
C. It is a good practice to reserve IP address for all APs?
We never noticed any potential impact reserving IP address in DHCP. Should be good practice.
In case of accidentally isoladted network and now two WIFI configurations are both running in the same network, when they see each other later, what will happen? Will they try to compete and make other AP to use the same config?
In that case Master APs will stay up and maintain it's own configuration until it's rebooted. However, we cannot guarantee the member APs, they might switch back and forth between two Masters, inheriting the current master config.