I'm a bit puzzled by the responses here. (he says six years later

)
In the EU version of Kellar's Keep, Belorn's Mine
is clearly a bonus Quest or side mission. It's not a fork in the path--just a brief detour. They don't both lead to Grin's Crag. I'm not sure why anyone would think that.
Unless otherwise specified, when you finish a Quest, you always move on to the next Quest in the booklet. And the Quests are arranged like so:
7 - The Eastern Passage
8 - Belorn's Mine
9 - The East Gate
10 - Grin's Crag
So it goes like this:
You play 'The Eastern Passage'. The parchment text says you're going to the East Gate. You get there via the SOUTH door (i.e. the door in the lower right corner of the map). Making sure to pick up Grin's map piece along the way. And indeed, if you take the south door, the Quest Notes tell you to play 'The East Gate'--which is Quest 9. You skip Quest 8, which is Belorn's Mine. But the Quests aren't numbered in the book, so there's no tip-off that you've skipped something (unless you're Morcar).
At no point is Belorn's Mine even mentioned or hinted at. It's pretty clear the Heroes aren't supposed to know it exists.
However, you can, if you wish, ignore Mentor's directions and take the mysterious NORTH door instead (top left of the map). Why? Because it's HeroQuest and exploring is what Heroes do best. If you do, surprise! The Notes say your next Quest is 8, Belorn's Mine. "Wait, we found a
gold mine?!" And indeed it has a pretty impressive treasure haul.
And when you complete Belorn's Mine, there's nothing about skipping ahead to Grin's Crag (Quest 10). Therefore you just play the next Quest in the book. Which is... Quest 9, 'The East Gate'. The same one you would have done if you'd taken the south door. Belorn's Mine even makes a point of steering you to the south again (the exit is at the bottom of the board). That way, whichever door you chose in Quest 7, you end up entering 'The East Gate' at the top of the board, because you're going south.
You don't get an easy shortcut to Grin's Crag. In fact, it's a long-cut. Uh... you know what I mean. You could have done the Quest Pack in nine Quests. But now it takes ten. You've delayed the journey for a sidequest in order to pick up some sweet loot. Which you had no way of knowing was there. Because you took the time to explore.
So in the EU version, Kellar's Keep 'officially' consists of nine Quests plus an optional bonus (Belorn's Mine) that some players might never find out about. The evil wizard player doesn't have to tell them. I reckon that's more interesting than forcing the players through all ten Quests.
For one thing, KK can be a slog to get through. Being able to limit it to nine adventures instead of ten helps.
For another, KK is sometimes criticised for being a bit dull compared to Return of the Witch Lord. And it's often true. But does Return of the Witch Lord have an optional bonus level hidden away in there? No it does not.
But most of all... one of the big appeals of HeroQuest is
exploration. It's what sets it apart from so many other dungeon crawls that just turn into endless linear monster-bashes, even today. And for exploration to work, you have to have odd things off the beaten path that not everyone is guaranteed to find. Kellar's Keep has the biggest one of all: an
entire Quest that you can miss if you don't explore. It's the biggest secret reward in all of EU HeroQuest. And they took that out for the NA version? Madness. Madness, I tell you!

by the way, Belorn's Mine is (as far as I recall) the only Quest to use the two trapdoor tiles. I wonder if that was deliberate, as a tantalising hint to players who missed Belorn's Mine that there was something else in Karak Varn they didn't see? "Hey, what are these things for? I don't remember them in any quests... aw man, what did we miss? Stop smirking, Mr Morcar!"