I don’t think the mobility is lack of hamstring length, just that I feel some irritation at the joint capsule when trying to bend the hip in flexion. Like a femoral acetabular thing. Any way to get around that?[/quote]
When you are doing your knee to chest stretch and/or your single leg leg press, how close to your centerline (a line that divides you into a left and right side) is your knee? Some people (myself included) have either tissue limitations (big thighs, big ribs/torso, etc…) or joint shape variations that will not allow them to maintain a lordotic arch while bringing the thigh to the centerline of the chest.
Try laying on your back and instead of bringing your knee straight up to your chest, bring it up at an angle to your chest (more like you are trying to bring it up to your nipple or even armpit) and see if that eliminates the irritation in the front of your hip (and allows you to keep your lordotic arch or at least a neutral spine).
Single leg leg press is also substantially more difficult to maintain a good spinal position during than a bilateral leg press IME; don’t know where you got the idea that it should be easier.