Having the mast bent to one side will make a slight difference in performance between port and starboard tacks.

YES!, you are correct about having the mast laying like that influencing the side bend...I didn't explain that part well. You only need to rotate it between adjustments so you can sight down the luff track to check that the diamond wires are even and the mast straight side to side. Rotate back again to check the displacement. Or - first get the mast straight by adjusting either diamond wire, then adjust the prebend by making equal adjustments side to side.

The structure of the rig depends on having adequate tension on the diamond wires - but there is usually a wide range of acceptable tension. As long as they are 'tight' when the boat is at rest on the beach they will provide adequate structural support for the mast while underway.