I think, like someone already said, the letters that Shuya received is letters or even words that Ayu said/wrote while waiting for him beside his bed, and for Shuya's letter which cant be sent is because he trying to communicate with Ayu but he just cant cos he still in coma. And about Shuya stuck in place with full of clocks, maybe it means time is all that matter for a people who is in coma, either he will wake up or just die, but time will decide for it. About Ayu in heaven-like place,well IMO it doesent relate with the story, it just a place for her to "sing". Sorry if this is a bad interpretation
