mostly harmless gets to be just about the most confusing thing in existance.
Way massive spoilers ahead. but it does bring some sense to what is written there... (this is pretty extensive so beware)
Basicly, it turns out that the mark 2 guide has -for whatever reason- orchestrated the entire flipping timeline.
manipulating the extradimensional beings into building the earth, manipulating Zaphod's psychiatrist and the Vogons into destroying the earth. manipulating ford into getting trapped, and befriending ford.
manipulating them into escaping, and Zaphod into stealing the heart of gold (so he could rescue them)
manipulating the Krikkit wars, so that a single ship could escape, so that it could reopen the time lock, so that they could barge the dude in the courtroom into receiving too much truth-syrum.
manipulating akrajag so that he gets killed so very thuroughly by ford (why? only thing i can think of is that so arthur can retrieve the bag he lost in greece)
errr
manipulating the ship so that they get stranded on prehistoric earth, so that arthur can meet up with fenny. (his prehistoric cave is in exactly the same place where fenchurch will live) and presumably so that all the lawyers and telemarketers and whatnot evolve into modern man (so that the question becomes garbled, wiping out another way that the mice can retrieve it)
manipulating ford into having a child (you know what i mean...) which becomes apparent as part of one of agrajags deaths.
manipulating him further into crashing on whatever planet it was, so that the the mark 2 guide could fall into the hands of random, while giving ford and arthur a way off the planet and back to earth.
manipulating the car drivers CD player into playing his least favorite track over and over so that he would accidently hit the eject button just as ford jumped out the 13th story window.
manipulating the aliens on persephone, so that they would destroy the earth.
and tie up all the lose ends, leaving no way for the meaning to life, the universe and everything could get out.
i reckon Thor orchestrated it all.
all of that is either;
A: said explicitly.
B:suggested
or C: subtley hinted at throughout the books or in interviews.
and there is more to boot.