You are right in your assumption of how truss members work they are axial force members and hence why as a engineer they are a joy to design especially when you can get them to work in tension only! (no buckling)
Trying to calculate the force induced into the 8 members can be done but its not easy and would require a bit of thought and free body diagrams. needless to say 2kg of equipment will not cause the member to come any where near failure nor flexure in my honest opinion.
2mm Aluminium tube with a Youngs modulus of approximately 80 GPa has nothing on CF tubing which is generally anywhere from 250-400 GPa depending on quality in the axis of the strand and about 10-15 GPa perpendicular to the strand.
Im personally still not convinced that a 4 post will give you a better outcome just due to geometry its a personal thing really but just for your piece of mind if you have a piece of scrap wood/steel put 4 screws/bolts though it. and try get each point touching the surface at any point in time you will find as you move one at least two will come free so you need to move two to get one adjustment. Do the same test with 3 and see the difference.
