I don't remember the book mentioning that navigators were giant mutant monsters. The mini-series did it but I think it was a pure Lynch Eraserhead invention.
The Third State Navigators are never seen if I remember correctly (the gas obscures them and no one but other Guild members truly know what they look like) but people are told that they look like giant fish-men or something like that. The mini-series version was accurate to the book (though a bit silly), the Lynch version is more disturbing and surreal.
Oops..I meant the navigator in the Children of Dune miniseries was accurate to the book. The weird man-bat thing in the original Dune miniseries...I have no idea where that came from.
There's an element of bullshit to everything the Guildsmen say, though. They know that they have a monopoly on space travel, so they build complicated mythologies about themselves to hide their technology. The best myth they have built is that of the "Defeated World", where defeated Landsraad houses can be taken for a fee to live away from the Empire. It could exist, or it could just be that they open the space doors and kick the dregs of the Landsraad out after they've admitted defeat.
OMG LET'S TALK ABOUT DUNE I WANT TO TALK ABOUT DUNE
I think the first three books make a perfect trilogy that stands very much on its own. My personal feeling is that 4 onwards was really unnecessary and shows Herbert really sort of struggling to over-complicate things. Leto II's plan is laid out in it's entirety by book 3 and we know where it will go, what will happen, and the predictions of the future are inescapably accurate. This leaves book 3 on a really cool note that leaves the future open to interpretation but you know that, no matter what, things will eventually turn out for the best for humanity. really didn't need more. It'd be like if Tolkien decided to make Lord of the Rings II which was mostly political intrigue.
What a coincidence! As it turns out, I just finished re-reading GEOD yesterday, and I think the sequels are great. In fact, GEOD is one of my favorites.
I only wish they'd included the other line from this scene I enjoyed. One of the escorts for the Navigator puts a translation device to his mouth and it speaks for him: "The Bene Gesserit witch must leave."
One of the things I enjoyed about that scene is that you can clearly hear the guild rep speak in his native tongue, and over that you hear the English translation of it. I always liked that small detail and it has always stuck out to me, especially after watching years of Star Trek and the magic English-speaking aliens (don't get me wrong, I like me some Star Trek as well).