Interesting post. My first thoughts:
1. From what I recall, John did for a second try to put pressure on Mary's wound. My thought was that - because he was a doctor - he immediately realised that there was nothing to be done. And so he focused instead of being close and giving comfort instead of futile efforts that would've only added to the pain (pushing on the wound would've hurt even more, for instance).
2. As for his attitude towards Sherlock in TLD, I think we've discussed this in other threads. John didn't really like Sherlock much at that point. If I remember correctly, I think Martin actually used the word "hate". And so all John saw, was a man again being lost to drugs, losing control. Just confirming for John why he is "right" in not spending any more time with Sherlock. I do think you can become rather jaded and cynical after a while, when close ones are drug addicts. Not saying it's right, only understandable.
3. The car-birth scene doesn't make sense either. Only thing I can think of is that it was done that way for comedy effect. Also, to put focus back on Sherlock (if John was in the back with Mary, Sherlock would have been reduced to being a chauffeur for the scene, and the focus would have been on John and Mary. The show is first and foremost about Sherlock as the main character, so instead of making it a dramatic scene about John and Mary, they made it into a comedic scene centering around Sherlock).