I guess I would have to see it to comprehend better but I do not understand how throwing across his body will cause the problem he is/was having UNLESS he was supinating rather than pronating at or after release. You also state, "He is getting better at producing a more forward (towards the catcher) momentum...." Where was his momentum going before?
More towards the middle of first base and home plate (throws right handed). I was told that before making the adjustments his throwing motion tended to put more strain on the inner elbow area. I am not an expert by any means, but after watching both ways on the "coaches eye", I could see is arm slot changed by making those corrections and nothing else.
Glenn, That is my opinion too and the further to the right of that line he is, the more his arm will tend to be outside as well. Then he will be throwing across his body as described. BUT, while that technique is counter productive I do not see the mechanics of how it will produce the type of elbow issues described without supination of the forearm.