I saw flux as being like static buildup of energy/electricity. Since you are in space, there is nowhere for the energy produced by weapons/systems/shields to go, so it just sits on your hull. "Flux venting" is actually just the venting of conductive gasses from the ship, allowing all of that energy/electrical buildup to be released without harming the ship or its crew. This would also explain why ships appear to be covered in electricity when they overload. The ship's built in capacitors overload, so they are forced to release the energy over the surface of the hull, rather than allowing the capacitors to degrade from storing to much energy, which could possibly malfunction and release all of that electricity inside of the ship, frying systems and cooking the crew inside.
As far as the difference between combat speed and travel speed, I always just thought that a ship had two sets of engines. One set of "maneuvering jets" for use in combat, allowing the ship to perform more agile movements, which are needed in combat. The second set of engines, the "travel" or "hyperdrive" engines allow the ship to move at extreme speeds at the cost of very little maneuverability outside of a set path. Thus, a Burn Drive, is just a very short pulse of the hyperdrive engines, allowing the ship to reach greater speeds at the cost of mobility. This would explain why ships use supplies to travel, besides stuff for the crew, the ship has to be able to constantly vent its flux to maintain travel speed for a long period of time. So supplies represent gas containers that would be used to vent flux
Just my 2 cents on it.
