The interface is standards compliant USB, so it communicates both ways. It's used for both downloading dive log data and updating the firmware. The computer comes with an adapter and cable, which is all that is needed to connect via USB. I'm not sure what you mean by a test harness, but if you are asking if we are providing help in interfacing with third party desktop or dive log applications, the answer is yes, we are encouraging it.
A test harness is usually a stub program that can be run on a computer without connecting to the actual dive computer. It returns results to a test application as if the computer was actually connected to a Cobalt. A test harness would simulate the maximums and minimums of a Cobalt so devs can test virtually every possibility without actually having to make each dive and every scenario.