While I would agree with you that the whole Daventry being another world concept isn't one of my least favorite aspects of the King's Quest Companion, considering even Roberta herself was very adament that Daventry was on earth 'long ago' in the games and interviews.
Well, ya, King's Quest is on earth. Daventry is very old city from a long time ago. It's in ruins now and people aren't quite sure exactly where it used to be. There are some archaeologists searching through the ruins, they think they know its Daventry. But its somewhere on Earth."-Roberta Williams, Mask of Eternity Talkspot Interview, 1998,
I will defend it only as such, that he was doing what Roberta did to a degree, that is to make references to classic literature and fairy tales, which is what King's Quest is all about.
In this case, the idea originates from a number of classic literature including Alice and Wonderland/Through the Looking Glass, Fantasia, Chronicles of Narnia, to some degree Wizard of Oz (which the book actually mentions some of these sources to be inspiration) which involve protagonists cross through portals to reach other lands.
It's an idea that Roberta even touched upon in King's Quest 7 with the pond leading to the world; Realm of Eldritch. Which is close to a concept right out of the Magician's Nephew. KQ7 also has the whole crossing over to the 'dream realm' aspect as well. With use of magic carpet to teleport into it directly.
It's really just a classic fantasy plot device, from late 1900's to early 20th century. You can also find it in more contemporary fantasy including the Xanth series, His Dark Materials, Dark Tower series, etc.
To Peter's credit, he is somewhat more original in that he makes the trip at least, one way. Once a person crosses over, there is no way to return. Also no one can directly communicate (or rather have a two-way communication) with individuals from the other realm. Not even Derek Karlavaegen directly communicates to any specific person (he wasn't trying to contact "Peter Spear" specifically in the story). He just types up things on the Eye Between the Worlds, and presses 'enter' (he didn't know anyone on the otherside)... The things he was typing up were randomly ending up in people's computer files (pre-internet references, it mentions modem communication to 'bulletin boards' though). He messages could be sent even if the modem was unplugged. Yes, this kinda makes him a spammer if it was on a modern computer! Again this communication is one way, there was no way for 'Peter" to reply back to Derek. There was no address to reply to, same with 'others' who were receiving messages in the same way. It didn't even appear to be in any 'mailbox' but just his folders or desktop. From Derek's POV he didn't seem to have any specific person he was contacting (he gives no direct knowledge of names of anyone from earth), but believes the device can be used to directly send stuff to readers in other world.
However, in one of the articles, the character Derek speculates that its possible that Manannan may have possibly had use for it to communicate with minions on the other side. But he could never figure out how to contact anyone directly.
Likewise, the Eye from Derek's POV may have occasionally picked up material from the other side, and through that he was able to learn more about what modern earth had become. But most of that information they received from occasionaly individual that withdrew to the world.