Xbee SX 868 questions


Before purchasing the Xbee SX 868 module I want to ask you some questions.
Here is my project :

I have on the first hand 3*3 switches. I want to connect all of them with an Xbee SX 868 to get their states. On the other hand, they will be 3 boards with 3 relays and one Xbee SX 868 on each board.
What I want to do is that when a switch is ON or OFF, the Xbee will send an order to the corresponding Xbee which is connected to the corresponding relay.
For example :

Xbee A (coordinator) :
Switche 1B = OFF
Switche 2B = OFF
Switche 3B = ON

Switche 1C = ON
Switche 2C = ON
Switche 3C = ON

Switche 1D = OFF
Switche 2D = OFF
Switche 3D = OFF

Xbee B (end device) :
relay 1 = OFF
relay 2 = OFF
relay 3 = ON

Xbee C (end device) :
relay 1 = ON
relay 2 = ON
relay 3 = ON

Xbee D (end device) :
relay 1 = OFF
relay 2 = OFF
relay 3 = OFF

I want this communication to be secured with AES 128 encryption and ACK.

I am aware of I/O line passing, but I am wondering if it’s possible to do such thing on a network with the api mode?

I am also wondering if it is necessary to use a microcontroller like an Arduino to do that? Or is it enough to use only the Xbee with line passing ?

Is it mandatory to have a pc attached to one xbee on the network or not? Because all of the examples showing Xbee network in api mode are using a PC attached to the coordinator device.

Thank you in advance…
I look forward to hearing from you soon.

Pierre OLIVA.