The version of this album that I read was the unauthorised completed version. I do offer my sincere apologies to the Herge estate, but I really could not read the published sketch version simply because it was clearly uncompleted. Herge began writing this in 1980 (his original idea of setting the final comic in an airport departure lounge was thankfully scrapped) but he unfortunately died before he could complete it. From reading what he originally wrote it is clear that there was a substantial amount of work needed to be done, particularly since parts of the completed version required substantial editing (for instance where did Tintin meet the informant?).
It appears that this was intended to be Tintin's final horah, and Herge had moved slightly in a new direction. In a way it is similar to Tintin and the Picaros, where Herge was attempting to wind up some of the unfinished plots, and also having a parade of all the characters (with the exception of Alcazar) through the story. This comic is also set in the world of Modern Art, something that I understand Herge was becoming ever more attracted to later in his life. It may be suggested that he was moving towards a post-modern viewpoint, but it appears that this is something that Herge rejected. While some of his comics are clearly modernist, and absurdist (such as The Castafiore Emerald) he was not a post-modernist author.
I will only deal with the completed parts here rather than look at the sections that other authors have written, particularly since it is glaringly obvious where somebody else has stepped in to complete the story. Herge and his estate made it clear that Tintin was not to continue after Herge's death. This, I believe, is a good thing, particularly since Tintin is Herge's creation, and Herge is really the only person who is able to get into the mind of Tintin and his companions.
Another change here is that Herge introduces a young, single, attractive female into the comic. While female characters have appeared (and Castafiore with regularity) in many cases they are thin on the ground and usually married (though the landlady also make appearances early on in the adventures). However, here we have a potential love interest for Tintin, which once again is moving away from Herge's norm. There is even a hint that both Tintin and the Captain may be attracted to her (as can be seen where the Captain takes her umbrella), however it is unclear where Herge was intending on taking this (and whether he was intending on actually making her a love interest). I did appreciate it that the completed comic did take it in that direction, however this was added after Herge's death.
One final thing that I discovered about unauthorised Tintin comics. There is one floating around (and available) called Tintin in Thailand. I have not read it (and have no intention of doing so) however I understand that this particular comic is actually quite obscene. The story is that this comic was going to be released as a 'recently discovered Herge manuscript' and was to be sold on the black market. However the police mounted a sting operation, arrested around 6 people involved in the production of the comic, and seized about 1000 copies of it. It appears that Herge's insistence that no Tintin be released after his death is taken very seriously in Belgium, particularly since copyright breeches generally do not attract sting operations. However I suspect also that there was more than just copyright issues with regards to this unauthorised comic (though I have since found it on the internet).
It also appears that Tintin fans also take this request very seriously, particularly since the Cult of Tintin aka Tintinologist, refuses to accept any fan based stories on their site, and other than a completed Tintin and Alpha-Art, and a number of speculative covers for other Tintin adventures, there is pretty much no other unauthorised comics available (with the exception of Tintin in Thailand, which, from what I understand, is an absolute travesty).