I know that scarabs cannot be created only harvested but i need questions on the following.

Is it possible to give the scarabs to a human player and then place them over a unit?
How large should the scarab harvesting area be?
How does the trigger work... Can i put the scarab over the unit without need to have it patrol?

The player ownership of a scarab is tied to the reaver that created the scarab. If you shoot a scarab and then give the reaver to player 1, the scarab will now be owned by player 1 and anything killed by it will be accounted towards player 1.

The scarab harvesting area only needs to be a couple of tiles wide if you're running hyper-triggers. Basically you set your reaver up to attack some target, like a mineral chunk powerup owned by an enemy player, and put the location between the reaver and the traget. It's important that the location doesn't overlap any part of the reaver, or it will detect the scarab before the reaver has fired it. Then use the trigger condition "Player x brings at least 1 scarab to [harvest location]" to find out when the scarab is ready for harvesting.

The easiest DDS system is to simply harvest the scarab, move it to it's intended explosion target, and then Move it back to an unplacable area - this will cause the scarab to stay where it is and detonate immediately. An unplacable area for a scarab is any water tile or land tile that is non-walkable or otherwise fully occupied (by buildings or other units), or a location that has it's co-ordinates off the edge of the map making it invalid to place any units there, including air units.

Ty this helped alot.