Depends on what your garlic goals are. I always waited until the scape made at least a 180 but preferably a full circle before pinching them. In my experience if done earlier than that they will oftentimes send up another one and thats just more work. No need to pinch them off low, I never did lower than waist height as bending over is work and I always figured that the green stalk left behind (stays green) must be photosynthesizing and contributing something.
Cut-um. You don't get propagation from those no why.
Not exactly right. You don't get sexual propagation, but you do get asexual propagation. Exact clones of the mother plant, along with rare mutations. If you are not interested in this years harvest but rather a tremendous increase in planting stock then leave the scapes intact. They will never make a real flower, but will go directly to little clone bulbils which may be planted like other seed garlic and in two seasons (first season they'll make fat rounds) will make bulbs containing multiple cloves. In my opinion, if you wait for the scape to straighten and elongate you're really not gaining anything as harvest (3 lowest leaves dead) is usually not too far off.
don't try to pull up the whole plant by hand but use a pitchfork or shovel to loosen the soil around them first. don't wash them or let them get wet. hang in a shady but breezy place to dry for a couple of weeks. then cut off the roots and leaves and gently brush off any loose dirt that hangs on to the bulbs.
DON"T PULL BY HAND is right. You'll "pop" the stem and the garlic will rot from the inside and you will be unhappy. I've got to add to the above: Don't dry them in the field after pulling, even for 15 minutes. Garlic will sunburn FAST. get it dug and into a polebuilding or other such structure to dry in the shade.