I don't understand where the "multiple Arduinos" comes into the picture. It seems there is only a need for one Arduino to send a message to a second Arduino.
If this was my project I think I would prefer a wired connection between the Arduinos for greater reliability. And with a wired connection it should be possible to implement the project with a single Arduino. In either case you still have the facility to add logic.
For the WiFi connection an ESP8266 would be much cheaper than a MKR1000. Indeed a single ESP8266 might be able to do the whole job on its own.
I must say, however, that if I was faced with a project that needs a web interface my first thought would be a Raspberry PI as the programming would be much simpler using Python.
...R