Keeping your elbow level is just fine! I teach the higher to lower draw simply because it helps folks "discover" the use of the back more easily in my opinion. Just a technique, nothing more.
Bone on bone... Easier to demonstrate than to describe. You might think of it this way. If you are doing push ups (just the old standard style) try stopping half way up. How long can you hold that position before you fatigue or start to shake? Now, if you push all the way up and straighten your arms so your weight is supported on the arm bones, how long can you hold? That is one demonstration of the advantage of bone on bone.
In the archery shot, if you have your bow arm straight (but not locked -- I'm a BIG advocate of the "not locked" elbow!) AND your shoulders (both of them) in line with your bow arm, you have created a "bone" structure that is "bridged" between the bow grip and the string hand. Many feel a lessening of the weight of the bow when this happens because you are not holding bow weight with the muscles but "bridging" it on bone. The same is true during the draw. IF you can think about "levering" your bones you actually can reduce the muscle strain by using muscle to move the bones and not just to pull the bow.
I grant that there is a "fine" distinction between the two, but once you "get it" it is actually easier to draw the bow and more solid. This, once learned, is the technique that allows a person to draw a heavier bow more comfortably.
Again, trying to put this into words is much more difficult than a one on one session where the movements and concepts are easier to convey.
I'll also mention, that if you think of your bones as a radius in a circle (especially the upper string arm (humerus) and string shoulder) where the shoulder end of the bone is the center of the circle, The bone swings, creating a circle at the elbow end. And, "circle" = "rotational" draw.
See if this helps a little.
Arne