While your description of riding "from the front back" is incorrect, you want to be riding from the back froward, seat into leg into hands, it sounds like your PC instructors are otherwise correct. To get him bending and flexing, pick up a conversational contact (i.e. Elbows at hips but elastic, soft following wrists, thumbs on top, and a closed grip) with his mouth, only enough contact to keep any slack out of the reins... NO PULLING BACK. You are holding hands with him. Don't worry about where his headset is, ever.
Maintain that feel of contact with your outside rein through your turns and circles (You'll need to allow that hand to follow the bend or you'll be too much in his face
), but allow your inside rein to give a little when he flexes and bends through the turn. As you're turning, be aware of your upper body - Don't drop your shoulder or collapse through your in-side or he will as well. Horses always mirror us. Use your legs as well to guide him through the turn - inside at the girth, outside slightly behind to control the hindquarters.
As he loosens and softens through his jaw, poll, and neck, you'll see "white lipstick" foam forming around his lips. This is a fair indicator that you're on the right track and that he's relaxing and loosening up, accepting the bit.
As far as the extension and collection question, that's actually rather higher level stuff, and I'll let someone more knowledgeable than myself go into the details. However, if you're just working on getting Buzz relaxed and supple, as your previous question indicates, you can work on lengthening and shortening his stride, or transitions within a gait. Use more of a driving seat, close your legs, and allow with your hands to ask Buzz to take a bigger stride at the same tempo at his normal working gait. Reverse to shorten - More of a holding seat, still closing legs, and hold slightly with hands (still no pulling, just a hold). Using a half halt to rebalance him for each lengthening and shortening helps oodles.
Best of luck!