2 questions. Why the guilt and why the second theory? Spoilers

I'm going with the theory that this is all Henry's 'dying dream'. As interesting as the other theory of Sam being traumatised by the event is, I don't think it makes as much sense as Henry's dying lucid dream. Didn't the director say that the reason Sam's trousers are too short is because that's how Henry see's them due to Sam crouching? So this is all from Henry's perspective right? How does that mesh with the second theory? It's one of the most obvious visual clues right from the beginning in my opinion. I was interested in re-watching this again recently and couldn't remember the name of it and actually asked my husband "What's that Ewan short pants movie called again?" Silly, but we kept mentioning it the first time we saw it.

I understand Henry feeling guilty and responsible, but he was totally consumed with guilt. Sam told him that he saw the accident, the tyre blow out, and that there was nothing he could have done and that he wasn't at fault. Lucid dreaming Henry doesn't appear to put much faith in this at all. I think Henry having his father see again was the only indication that he had somewhat dealt with his guilt and accepted he wasn't responsible. Or was that just a metaphor regarding the impending realisation - Once was blind, but now can see...?