Have you allowed for a windstorm where the trees may be moving in opposite directions to each other?
Temperature affects the length of the wire rope, have you allowed for maximum and minimum temperatures?
You want some sort of shock absorption built in too. Old antennas used porcelain blocks for joining cables, the porcelain would shatter under unexpected loads giving the cables a chunk of extra slack to avoid their collapse by stretching beyond limits.
May I suggest you find an engineer to look over your design? I'm not one, but can think of a few ways for this to go wrong already including the clamps failing etc.
Suspension bridges are close to what you are building - read up on the design criteria for these. Seat of the pants design may give you another Tacoma bridge...
John :-#)#
PS, it looks like a lot of fun though!