I think I need an explanation of the ending of the first book. I understand that Dirk became the "acquaintance" who interrupted Coleridge while he was in the process of writing Kubla Khan, and that in doing so he shook the ghost out of his body, but how did that save humanity? What did the ghost plan to do? And what does any of it have to do with shoving Bach into the timeline? And what on Earth happened to the poor bastards delivering Richard's sofa when Reg's flat left that point in space/time?
It saved humanity because Kubla Khan, in the original timeline, was much longer and contained information about how to repair the ghost's spaceship. When Dirk destroyed it, the information was lost; so the ghost could return to the ship, but not repair it.
The ghost planned to return to the spaceship and repair it.
Bach published the music they recovered from the mothership.
The sofa thing worked like this: In the normal layout of the staircase, the men couldn't get the sofa anywhere near Richard's door. When Reg's flat was present and his door was open, that added enough space to get the sofa a bit further up the stairs. Once the sofa had gone past, Reg's flat disappeared. The sofa and the delivery men were still on the staircase, but now the sofa couldn't go forwards or back. Presumably the men eventually gave up and went home.
The ghost planned, not to repair the ship, but to prevent the accident that damaged it in the first place. This would have wiped out humanity, because the accident was also the event that began life on Earth.
Yes, but how would Coleridge not finishing Kubla Khan fix anything. The ghost still knows what to do.
Without Kubla Khan, Michael Wenton-Weakes doesn't have the right mindset to be possessed to do such things, I mean, it wouldn't occur to him that it was doable, and the possession wouldn't take.
The way I see it, Kubla Khan part 2 was not so much (or at least, not merely) a set of instructions for fixing the ship, as it is a musing on returning to the past and correcting ones mistakes, and the desirability of doing so. Whatever it is, it seems to be a tipping point in allowing possession. After giving up on Reg, who is too opposed to the idea, and the Monk, whose ability to believe anything might have removed the need for Coleridge, if it had been workng properly, the Ghost settles on Richard, who has just listened to KK in its entirety. Unfortunately, he just doesn't have the right personality to go through with it either, at which point the Ghost fortuitously encounters Michael, who is already obsessed with the idea of righting a wrong done in the past (not to mention, apparently full of a deep if subconscious self-loathing), and just needs a gentle nudge towards Coleridge to make him perfect. Removing KK from the equation meant that his possession wouldn't work, or at least would have taken a lot longer, by which point the Time Machine was no longer working.
As for the Bach music, it appears to be some kind of blueprint for the perfect world that the aliens intended to create. I assumed that Reg fabricated Bach's entire existance as a means of saving some of the music from the ship. Adams apparently genuinely regarded Bach's music as something transcendant and close to divine.
Related, but not quite the same: I've always been bothered by the Timey-Wimey Ball aspect; are we in a Stable Time Loop (Richard's sofa suggests yes) or not (nearly everything else suggests no)?
Stable time loops can arise from a system allowing one to change the past.
Everything in the Dirk Gently series is said to be relevant. But having read it for the umpteenth time, just what eactly is the significance of the Monk and of Gordon Way's ghost? Sure, the monk was brought to Earth by the alien Ghost as a possible candidate for possession, only to find it was malfunctioning and useless, when it then goes on to kill Gordon as a result of misunderstanding a throwaway remark. But why do we then have several chapters of them mooching around, not doing very much, until Gordon finally gets to make his phone call and Reg takes the Monk back where it came from?
Gordon's ghost is what puts Dirk onto the idea of possession in the first place - they "met" in Richard's office, remember?
Also, Gordon's ghost informs Susan (and thus Richard and Dirk by proxy) that Albert Ross was murdered, which allowed Dirk to figure out exactly what the ramifications of their recent actions were.