To stretch the pec minor preferentially bring the arm back behind you so that the shoulder goes into extension and use a doorway or whatever to stretch that way. That will give a bit more of a stretch to the pec minor. Try to bring the scapula into retraction while you do this. I've tried doing this in a corner as well but I don't find that to work well. But you don't need to feel a really deep stretch.
The BEST way is more of a manual passive stretch for the pec minor involving a partner. This would be a typical thing a therapist would do for you. To do that you would lie on a foam roller with the foam roller vertical instead of horizontal…so that it lines up along your spine and between your shoulder blades. A therapist would probably use one of those half foam rollers for this. I use a whole roller. You can also use a rolled up towel for this…a big towel. DON'T do it with a PVC pipe! Put something under the head as well for support.
So you lie on the roller or towel that way and you bring your scapula into retraction and depression as much as you can. Your partner then pushes down, using the heel of the hand, and in the direction of retraction on the front portion at the corocoid process of the scapula. The partner should NOT be putting pressure directly on the humeral head. Using the heel of the hand on the corocoid process the partner pushes down and slightly toward the humeral head to force a stretch of the pec minor. You should feel this stretch in the chest but if the pressure from the hand hurts that is a no-no.
Here is a picture of the approximate area to push down on: Corocoid Process
And here is a picture of the strech, but without the roller or towel: Partner Pec Minor Stretch
Knock-knees are something you should check out with a professional. I wouldn't like to get involved with that. Of course it can be acquired a may have something to do with hip medial rotation. But it can also be associated with foot position and other things. And if there were a structural problem with the knee we could be poking our nose in places it doesn't belong. For simply postural corrections I can help but for this kind of thing I am not qualified.
Here is a Pec Minor Tightness, Stretching and Release video. Recognize that the table maneuver he is doing is not the stretch I have described above but an attempt to manually release the pec minor muscle itself…which is whey he is using the knuckles or finger and manipulating the arm to release tension.
You can do self work on the area with a tennis ball or a smaller bouncy ball to really hone it. Or with a theracane. You can use your thumb as well to some extent.