Well I've finished it now and I enjoyed it right to the end. Throughout the book, a clear distinction is made between real events and those which have been dramatised to some extent by the use of the two fonts. This subtle device doesn't intrude on the narrative unduly, but is an honest way of acknowledging when a part of the story had been embellished/surmised/made up.
As for the author not seeing Hannet for a few years before his death, I see that as just another story of someone who is forced to remove himself from a friend's company in order to break a destructive cycle.
The fact that Sharpe is working, clean and, well, alive, shows that sometimes the only way to stop a destructive relationship from dragging you down is to remove yourself entirely.
It wouldn't have been much of a book if he's ended it in 1981, or whenever they stopped hanging around together, so it was quite right for him to finish the story in his fictionalised style. He doesn't labour it; only a couple of chapters painting a picture of the chaos surrounding Hannet's decline and the sorry portrait of his family at the funeral, surrounded by crazies and half-friends and parasites who blighted him in life.
The "Who Killed..." device is a little pointless, as is the speculation at the end of the book. You really don't have to look very far to find a cause of death for a 22 stone drug addict whatever their age. You rarely hear about them dying whilst skydiving, put it that way.
The end result is a fascinating and very disturbing tale of the decline of several drug users, and a rare insight into Hannet's life and ways. If Sharpe wasn't all that close to Hannet, he's done a great job of telling a story about someone who was.