Xbee S3B (Arduino) Best network layout question?


I am new to Xbee and have a simple project to implement using it with Arduino. The project is simply to build a laser-beam breaker that remotely (over a distance of about 6km) notifies two different alarm systems.
For example, the Laser detector will sense there is a break in the beam, then need to send a signal state (only one pin either HIGH or LOW) to two different remote devices, each with an alarm, every few milliseconds.

My question comes down to: Which device/node should be the coordinator?

The way I currently have it “designed” is for the Arduino-connected Xbee (S3B Pro 900Mhz) where the Laser sensor is, is the coordinator and is has a bigger antenna attached. The two other Xbees that are connected to sirens, are set as routers and have standard low-range antennas attached, but have no Arduinos attached (the Xbee is directly connected to a relay connected to a siren).
The reasoning behind this design is that if the coordinator is set to be the xbee at the laser detector, then there is no need to add Arduino to the two xbees that are connected to sirens ( the digital output command can be used). The coordinator can simply push out the state of the laser sensor to all other xbees directly without needing message confirmations over the long range.

Is this design correct?

When I read on the internet, it seems it is suggested rather that the coordinator should be a back-end device without user interface, which makes sense, but would mean that I need to add extra Arduinos on the xbees that are connected to the sirens (which is overkill since no logic code is needed); would have extra latency (since the router first sends to coordinator then relays it out to the other routers instead of just sending it directly); and would need to possibly add a 3rd xbee to the network so that it can act as the dedicated coordinator that does not have any sort of user interactions; instead of the only Coordinator needing a high-gain antenna to reach the required distance to send a message that does not need confirmation status, all the xbees will need high gain antennas to be able to both send and receive data in the network.

Any tips or suggestions would be much appreciated.

Leobot, as far as I am concerned, I would set the Coordinator at the Arduino as well.