How to calculate surface area based on points of contact from a contact sensor

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Currently I a have world file that consists of a simple gripper and a t-handle shape floating in space. In the definitions for the models I defined contact sensors for each link that I wanted to detect collisions on of the simple gripper. Finally I wrote a model control plugin that accessed the simple gripper model, used gazebo subscriber callback functions to look at contact data from the different contact sensors, and I am able to look at each collision and the points of contact based of that collision. Similarly as this tutorial describes (http://gazebosim.org/tutorials?tut=co...).

My questions are the following?

1. Is there any pre-made functionality to calculate surface area of the collision as opposed to just looking at points of contact?
2. Does each gazebo callback definition automatically create a separate thread to act asynchronously, because I have no understanding on whether all the callback queues and other functions are all running through one thread, versus multiple threads.
3. Are there better sensors for detecting collisions on a gripper, versus a contact sensor?

Thanks Francis123