What shape Geofences can be stored on the device? Rectangles, Polygons or Freehand? 

Polygons - for more information see Geofence Downloads to Device

How is Driver ID validation done, on the device, or in the cloud/on the server?

This is done on a device level. If it was done in the cloud, and the device was out of coverage, you would not be able to start a vehicle if it wasn't able to retrieve the driver list from the server.
However, the Driver ID list is set up on Telematics Guru (the cloud server) and is periodically pushed down to the device and stored there.
See Driver Lists for more information. 

How many Driver ID's can be stored on the G120 Driver List?

See Driver Lists, 4000 - 16000 depending on layout. 

How do you manage drivers/operators on the device?
See:

• Immobilisation and Driver ID
• Driver ID, Immobilisation Methods with Powered Devices

How do you manage one trip with multiple drivers?

Every time a driver ID card/tag is scanned, a 'Tag Scanned' log is generated and sent to the server.
So even mid trip, a log is generated. It is up to the server as to how this is handled.

How does driver behaviour (harsh acceleration, braking, cornering) extend to off-road driving, or is it optimised for on-road? 

Harsh driving events aren't 'optimised' for off or on-road driving. The device will simply detect any acceleration that is greater than the threshold set in device parameters. It is difficult to distinguish between harsh driving behaviour and simply driving on rough, corrugated roads, as in terms of the acceleration forces experienced - they appear the same to the accelerometer! There is not much that can be done about this, the laws of physics are quite fixed and unchangeable!

That being said, our devices still have plenty of smarts! See here for more information:
Harsh Event Setup

For over speed alerts - does the device check the actual posted speed by road? 

The device itself does not, as there is no way to store all this data on the device!

This can be done later with the data on the server for reporting processes. This is implemented in Telematics Guru - see here: Speed Monitoring in Telematics Guru

The Geofence Download to Device functionality can be used to set a speed limit for an area. For example when on a work site - the speed limit is 40km/h, and the device should buzz at the user and flash lights when exceeding this. But when you get back out on the open road, the buzzing stops. 

How many BT tags can be detected by the G120 at once?

The maximum number of tags the gateways can support is 512. This is the maximum tag list size on these devices. As for the practical maximum, that is dependant on many factors. Imagine you were in a room of crowded people that were all shouting, and you were trying to listen to every single one of them. Inevitably, the people closer to you will drown out the people farther away from you, but if you listen for long enough you may hear the people farther away when those closest to you are taking a breath. The same is true for Bluetooth, and the maximum number of devices that could be picked up by a gateway is dependant on:

• The beaconing rate - beacons are less likely to clash if they beacon less frequently, but that also means if they do clash then you have to wait longer for a chance to pick them up again
• Background noise - other bluetooth and wifi devices may interfere with signals
• The scan length - the longer the scan length, the less of a problem clashing is. For example, if tags are beaconing every 1s, and the scan length is 15 second, then there are 15 chances for the beacon to get through to the scanner.