>>11321693
>but i do it with shoulders on bed, so it's not ideally straight
You may need to find something knee level so you're horizontal when fully extended. You could try bending knees more, but then you'll start feeling it in the quads
A chair or a couch could work also (though you want something sturdy obviously) buying a bench may not be a bad idea if you'd prefer to workout from home, but a gym will give you access to abduction machines which are better than bands (I've been working out with resistance bands mostly and increasing the resistance with them doesn't seem to increase my lifts at the gym very much, but does at least maintain them)

>knees crack with sound
This could also be a result of the back rest being too high
It may not be an issue, my knees often crack from squats. Maybe try stretching the adductors (inner thigh) beforehand. Maybe also some hip internal and external rotation stretches and exercises could help. Maybe some deep squats to warm up the knees a bit

>hitting a wall even with resistance band in exercises like clamshell
If you mean increasing the resistance suddenly gets too hard, you could try assisting with your arms and trying to slowly move back together (doubling up bands so pulls closer) doing it seated as a regular hip abductions may be easier for this
If you mean it getting too easy, you could still try doubling it and doing forced, slow eccentrics like I said above. A different workout would help though since you do want something that applies load not at just peak tension. If you could tie weight to your leg (above the ankle) and lay on your side on a bed or other raised surface, then you could do a straight legged hip abduction with full range of motion and good tension at both the stretch (elevating your other leg so it's out of the way) and contraction. That will be as good as any gym machine for abductors and maybe you could tie the weight above the knee if you're concerned about putting tension across it like that