You can put a (firm) stretchy band or a towel around your ankle to give you more reach for that quad stretch. You can do it side-lying which will let you stop worrying about balance while you shift your heel around to target the different muscles of the quads.
All the other quad stretches I know are deeper. I would start there ... except for what you said about swelling. Can you be more specific? Do you have a Baker's cyst? Has a doctor looked at the swelling?
Also, do you really feel the tightness *behind* your knee?