The tip only stays in contact with the cb for .001 seconds. What the cue does after that is immaterial. Most pros do not move their elbow until after contact with the cb.
Look at your diagram again real close. On the no elbow drop one, the cue remains level for several inches before contact to several inches after contact. That is all that is needed. The cue is level upon contact.
By dropping your elbow before contact, you require the co-ordination of a lot more muscles to keep that cue straight. Thereby making it much easier for something to go wrong. Best to move as little as possible, which the pendulum or no elbow drop stroke does.
After the contact with the cb, it doesn't matter for the shot what you do with the arm. It has no effect on the shot. Some prefer to drop their elbow after contact because it feels more natural to them. Some prefer to not drop the elbow and just continue the stroke to it's natural finish. The key is to not drop before contact for the most reliable stroke.
As far as the bridge length, with your arm at 90 degrees to the cue, your tip should be right at the cb. If you do this, then how far you bridge from the cb doesn't matter technically. But, the farther away you are, the easier for little tiny movements in your back hand to affect where you actually hit the cb.
Neil,
It is obvious you have spent much time thinking about your fundamentals and have decided what is correct for you. I don't disagree with anything you have to say (it makes allot of sense) except that not everyone will fit your definition of a correct stroke. If someone was learning pool from the start, following your advice would be beneficial.
I have allot of elbow movement in my stroke but I wouldn't say it is a good or a bad thing. I never even thought about my elbow when i was learning. From my perspective the only thing that really matters is that you have an accurate repeatable stroke and true command of your cue ball.
Dudley