When I was the advisor for the Urban Forestry Club at the U of Minn. we set up a swing a few times. Like yours the trees were at the top but leaned downhill. The swing's arc took the rider out over a sidewalk. It was surprising how few students walking on the sidewalk even looked at what we were doing. Lots of hoot n hollar!
A load releasing hitch works too. I like the Mariner's Hitch
Hey fun seeing that - I've put up an 8m swing for our kids in our 120+yr old Oak. I can give the kids a good swing on it (about 20' at the bottom end of the swing) but when we have older 'kids' about I get the climbing ropes and tackle out and a trigger snap. Loads of fun and the suspense before the drop is priceless