setting up the sling so the cuff is symmetrical, i.e. each side of the strap pulls evenly on the cuff, and rotating the cuff so the strap is over your elbow, and the cuff itself pulls against the back of your arm if often recommended, because it does work to reduce pressure on the brachial artery for many shooters.
Since these aren't effective for you, could the sling be slipping around your arm? The cuff should not be really tight around your arm, and you do need space at the front so blood can circulate to your hand, but if it's dragging round you can fix it to the keeper. The rotation of the cuff needs to be precisely aligned, or the sling will pull unevenly on the jacket, dragging it down. Moving the handstop back, probably wouldn't alter where the sling pulled from if it was properly aligned; the angel of your arm to the rifle would change, but the sling should stay in the same place on your arm. What you found was that you effectively lengthened the sling, and lost tension.
How well does your jacket fit? Could it be slipping down your shoulder?
Do you have enough padding under the sling? In my experience cheaper jackets skimp on the rubber, and the pad doesn't extend high enough, so a high sling sits over the pad on the canvas. You may then need a thicker underlayer to compensate; a T Shirt won't cut it anyway. I'd also check whether the jacket is creasing under the sling; off-the-peg jackets tend to have wide sleeves, so there may be excess material to fold.
Thinking of sling height, have you tried varying the height of the sling? As I mentioned above, the rubber sleeve patches are often too low, but you can place the sling above the pad, which may move the sling further from the artery.
How is sling tension? Too much tension can make the cuff bite your arm. It's not true that more sling tension is always good, once the position is tight and stable.
You may consider the position more generally. Is your left arm straight, with the elbow beside the rifle, not under it? Does the angle of the upper arm, roughly mirror the fore-arm? I see many shooters hunched over the rifle, because the butt/handstop haven't been properly adjusted.