Maybe a way to approach the master valve/concurrent waterings would be to allow any connection to give a list of valves that should be turned on. This way you don’t have to care about master valves, etc as it is the responsibility of whatever is sending the commands not you.
An added bonus would be to say turn this list of valves on for x seconds. That way they will turn off of their own accord if the turn off command never arrives. (remote wireless valves like this would be very cool. Commercial ones are extremely expensive!)