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I am not an engineer so don't know exactly what you should do for the lengthened drawbar. I think I would try one link more (6) under tension and see what happens. If that makes it worse, go back to 5. Just realize that that extension gives the trailer more leverage. You might be better putting it back in the other hole and just watching carefully how tight you jacknife the trailer when backing into a campsite.
As far as the slight push from passing vehicles, you will most likely always have some of that. I even get a bit of it with my Hensley but it does not require steering correction. Don't expect to eliminate that altogether.
Barney