I have found all of the above methods (planks under heels, wide stance, weighted squat for stretching) to be highly successful, even with some older men who could not squat as far as thighs parallel to the ground, yet after a few months of using these strategies at least twice a week, they have eventually managed to do overhead squats (snatch style) and full squat snatches without knee injury.
Interestingly, three of these older men, a Masters tennis player, a former sprinter and a bodybuilder had all been receiving physio treatment for more than 3 months to resolve knee pain (chondromalacia patellae or peripatellar pain syndrome) and were warned against any form of squatting. Instead they had to do the usual isokinetic knee extensions to little avail - one even used transdermal anti-inflammatory patches to handle the pain. After following the above strategies, all of them managed to do full range squats without pain or any medical treatment and return to fairly vigorous competition, so, at least in their case, full range, pain-free squatting (with something like 50% more than bodyweight) not only became possible, but it enabled them to improve on previous sporting achievements without further recourse to medical treatment.