advice re best exercises for strength knee ligaments,especially collaterals.

Once ligaments are stretched (sprain) they do not quickly return to preinjury length/tension. So exercise in itself does not heal a ligament sprain. What exercises can do is ensure ideal joint alignment and muscle function since both of these components becomes compromised with ligament injuries. The grade/severity of the sprain is also important to take into consideration. A knee brace can be essential for significant instability. Ideally a whole body biomechanical eval can determine your static and dynamic postural alignment - how well you stand, move, balance etc.