If you find clams difficult then pillow may well help, so don't have to go through whole ROM, you can just work on small movements. Think recommended was 10x10sec holds to work low threshold, need to work this area to prevent fatiguing when going on long run. Try not to use wall as this may lead to compensations, I had the same problem and found using a partner really help to get technique right in first place and monitor I was not cheating as I progressed through the exercises.

Gluts / hams...can start with knee flexed to isolate gluts more but aim is to initiate gluts then hams and in functional way...thus do need to do with straight leg at some point. (also make sure not activating lumbar muscles)

Nice exercise to try to see if you can dissociate lumbar from hams....
Sit on edge of plinth (feet flat on floor), use hands in small of back, tilt back and forth to find neutral, (make sure sitting tall) try and straighten your leg slowly and monitor what happens in lumbar area....tendancy is to slump if unable to dissociate.

Good luck, let me know how you get on.

Kinetic Control Courses really good for this sort of control work.