The other day, I was reading through a thread you started last year on this subject, and Xvimbi's advice that the angle between the upper arm and the forearm should be about 110° with the fingers positioned on the white keys seems to put me in about the right place. However, assessing this angle every time one sits at the piano is a chore, so I noticed that if my kneecaps are just under the keyboard everything else will follow. Of course, everyone is a different size and shape, but having a straightforward point of reference means you will be able to easily find the correct position easily every time you play.
Regarding bringing one's elbows in front of the body, I find that new difficulties emerge. If I fix my forehand and hand in relation to my upper arm, and then move my upper arm at the shoulder in front of and away from the body, I notice that the forearm and hand tilts accordingly. This tilt can be corrected by rotating the forearm, but (for me at least) when the elbow is brought in front of the body the forearm is already fully rotated, so the thumb ends up being higher up in relation to the hand than I would want. Correcting this with further rotation seems to negate any advantages of avoiding tension and stress that might have existed by adopting this position in the first place. I therefore conclude that whilst this position may have its uses, I don't believe that it should play as prominent a part as you might be thinking. So, if you are sitting further away than you want to for the purposes of getting your elbows comfortably in front of you, I would suggest reconsidering, and maybe make this kind of move more of an exception than the rule. Hopefully that makes sense.
Regarding body movements, I can see completely why your teachers would object to Lang Lang-style body movements (I must confess to really having no idea how Lang Lang looks when he plays, as I've never seen him, but based on others' comments, I'm imagining that this is the best way to convey what I'm thinking of here), but if your body movements are solely made for technical reasons you should have no problem with your teacher. If you do, explain why you are moving this way, and if he disagrees ask him to show you what he would do instead. Maybe you will be shown an alternative way of getting around the problem, or maybe you will end up convincing your teacher. As far as exams go, I'd have thought how you move is of no consequence compared to how you sound.
By the way, I'm in the same boat as you in that I'm also looking into the movements I make, so I don't have every aspect of technique clear in my own mind just yet, but I hope that my thoughts so far help you a little.
Richard.