My theory of what the writers intended in that scene comes from the fact that they matched what Shane said about the gunfire (not loud) to how they actually produced that scene (with gunfire that was not loud).
They could have had the gunfire be loud, and therefore cast doubt on Shane's story. What's the purpose of having Shane tell that story at all, really? Just to create angst with the love triangle? I think there's a larger agenda afoot.
I don't recall Shane saying his hearing had been deadend. If the writers wanted us to know that detail, they would have included it in the dialogue (perhaps having another character mention it as a possibility, or show Shane being exposed to a very loud sound right before the hospital scene). Ditto about a reduced heartbeat, although I'm not sure how they could have dramatized that notion one way or the other without destroying the ambiguity in Shane's story.