Yes, IMU has a '328 - you can see on the picture and the 2nd page of the schematic
The RS485 shield you found would work; unfortunately the IMU does not use Arduino footprint - you should be able to find a smaller RS485 type adapter to wire up to the IMU.
A full duplex version of something like this
I2C Mux with SensorStick - do you have a link to Sensor Stick? I am not familiar with it.
Do you mean that Serial multiplexing is necessary, because Xbee already uses one pair of TX and RX?
Yes.
How good is your coding? If multiplexed, the 328 on the IMU would need some reprogramming to set up a protocol to let each one know when to send data out.
You might be better off with a Serial Port mux so the Mega would control which IMU it was talking to vs modifying the IMU code.
Two quad-AND gates and a quad-OR gates would do that:
