Either. Mine is meant to go across the top of the rump and that's where I've always used it but I found on my Camptown harness I couldn't place it correctly there so I used it above the crupper. It seems to me a strap across the top of the rump stops the horse by main force and must be very strong while one above the crupper would stop the horse more by the fact they'd tuck their rumps and scoot after hitting it. (I'm not sure I want an already fractious horse doing that.) I also prefer a top of the rump one as it sits above the breeching and didn't disturb the hair as far as I could tell. I noticed the one above the crupper rubbed a lot of winter hair between it and the crupper buckles.
It's very important to make sure the strap can't slip forward past the point of the hip but other than that I think just about anywhere would work. Iowa Valley Carriage makes a good one, so does Camptown.
Theoretically. It's usually a pretty wide strap and might not fit. Most of them have loops on the top of the kickstrap for running the backstrap through; I put mine over top of the backstrap between the two hipstrap keepers and ran the loose end of the backstrap through the kickstrap loop then down through another keeper to secure it. All I had to do to release it was pull the loose end of the backstrap out of one keeper, lift the kickstrap off, then tuck the backstrap in again. Easy!
I do think the further forward it is the less shifting it's going to do. I mean, think about it. The horse should be moving their rump almost completely from one side to the other when correctly bending and if you've got the kickstrap fixed in the middle then the wider that part of the shafts, the more it's going to drag the breeching from side to side. Yuck! On a narrow set of wooden shafts it's probably not as much of an issue but on roomy bent shafts further forward is better. Lowered shafts represent a similar problem.
I certainly wouldn't discount a crupper kickstrap but know your rig and your horse and what will work for you. Kody's crupper is pretty narrow so the rubbing came from the kickstrap settling too far down and pressing against the crupper buckles. A horse with a flatter croup or wider split in his crupper straps would have been fine.