I have not tested it yet on the water ; ok, for your questions :
- if you have the adjustment line (elastic) attached at the correct line inside (attached directly to the red line on your picture close to the other extremity), the tension of the elastic should not depend on the adjustment knot: the elastic will be released from the same length as the adjustment line will be pulled)
- In my case I will try first with knots enough spaced as you said which should fit my needs (about 6 cm) ; however you can add intermediaite knots if you want, it should not be a problem if you pull right in the axis of the bar), to be tested.
- I suggest you swap the path between each side of the black&white rope around the pin on your picture; the sliding of knots will be easier ( I started like your picture but then I swaped) : indeed it will leave more space to the knots under tension to slide (useful for light winds with low rear line tension, while if the sliding is worse on the slack part with elastic, it is not that Important during a ride, you can fix it later on in the ride if it never slides)
Here is a skematic of the principle that a friend did to understand my setting :