Thanks, MonstorMotor! This thread is mostly about the German version and it took me some time to digest all the information. My German 1st Edition rulebook is product number 4271D390. This means it is the censored German 1st Edition, that came out in 1990 after the uncensored German 1st Edition 4271D489. According to this thread the censored version was the same as the uncensored one rulewise, but all text passages that might have been harmful for a German kid were replaced. E.g. you have to "capture" Ulag by reducing his body points to zero, and not kill him. And of course there is the ridiculous German epilogue to the base quests (Mentor's German sermon), in which Mentor tells the heroes that true heroes should not use violence to solve their problems, and if they did, Mentor would no longer be their friend.
The censorhsip was probably done to prevent the
BPjM from taking HeroQuest off the shelves and forcing retailers to sell it "under the counter". This was done for many German versions of video games back then, like the green blood in Half-Life, but it was a threat to all published media.
I would really like to see the uncensored version, and if the rules were unchanged. The German 2nd edition (4271D590) is very close to the UK version (except the censorship), but the German 1st edition has many passages rewritten compared to the UK 1st edition. For example the rule that you should not keep ordinary treasure cards is missing. In comparison the French 1st Edition (4271F1089, available as scan on Olde Inn) is a direct translation of the UK 1st edition.
For our current discussion about placement the rules are as follows:
The "Moving Around the Board" rule would be translated as:
The first player to enter a new room or passage must interrupt his move. This gives the Evil Wizard Player time to consult the quest book and place all visible items in this room or passage onto the board.
The "Opening Doors" rule would be translated as:
When you open a door, you see everything in the room or passage behind.
The "keep checking" rule in the chapter about the Maze is missing:
"As the players move their models, the evil wizard player must keep checking the map and put out any pieces that become visible."The rule under "Setting up a Quest" would be translated as:
No pieces outside this room are placed on the board. This happens little by little when the heroes can look into the other rooms or passages.
While not perfect, the German placement rules are much clearer. You place on the board everything that is visible. You can look into rooms or passages. This probably means "around the corner" and through an open door, because you can see everything behind an open door. There is a contradiction to the spell-casting visibility, so it is clear that placement visibility is a completely different thing.
I would not say that the German version is the better version of UK 1st edition, but at least it is one interpretation, and it is identical to mine. So finally I have found the reason why my interpretation is so evident to me. It is simply the German 1st Edition rules that we always used. And that's why it feels awkward to me to use spell-casting visibilty for placement.
Now back to the problem about the boundaries of passages:
Passages may be one or two squares wide.
This sounds to me that at least the 2-square passage is a different one than the neighboring 1-square passages.
Another hint is found in
Kellar's Keep Mission 3:
B The traps in this passage are so well hidden that any player who searches for traps will only find the next trap down the passage.
There are 2 letter B marks, so you would think that these 2 are different passages.
The same for
Wizards of Morcar Mission 4 and the Orcs with the bow (letter A).
A All the Orcs in these corridors are armed with bows and may make ranged attacks with 2 attack dice.
And also in
Against the Ogre Horde Mission 3 they are different corridors:
B Whenever there are any adventurers in any of these corridors during the Evil Wizard player’s turn, he must roll one normal die.
But then, in
Return of the Witch Lord Mission 3, 3 corridors are considered one:
A The shaded corridor is haunted by the Death Mist...
So the missing definition of corridor boundaries even caused inconsistencies in the quests! For placement both definitions would be fine I think. You could argue that the monsters around the corner can hear the heroes and are thus activated. On the other hand it feels odd to reveal the complete
Spiral Passage at once, but hey, it is called the Spiral
Passage (singular)

.
Regarding the "opening doors by stepping in" rule alternative. We have just tried this rule for Wizards of Morcar, to make the fire burst traps work at all. But after a while we forgot about it and switched back to "openig doors while standing still". There is even one mission (
Mission 3) where an Orc is standing right behind the door, so this rule can even lead to the pass through rock dilemma. I think this rule is just not practical and I understand why they dropped it (if it has ever been around anyway). So while this rule removes the discrepancies between our interpretations, it is only hypothetical and there is even a rule that explicitely allows opening doors while standing still.
The

and

problem only happens in 2 square wide corridors. In a 1 square wide corridor we would have the same situation only when the dwarf is right next to the new corridor (1 square north in the diagram). In this case it feels right to reveal the complete corridor (the dwarf looks around the corner), although looking down a corridor is not covered by UK 1st Edition (only German 1st Edition).
You could argue that although the goblin is shown on the quest map on this square, it doesn't necessarily mean that it is there all the time. Maybe it just arrives when the dwarf reaches the junction, maybe it lay in ambush. So the dwarf can target the goblin square while it is not revealed, but there is no goblin on that square yet.
If you use LOS for revealing the goblin, and let's say on the X there is an Orc, then the goblin is allowed to attack the dwarf on Zargon's turn, but the Orc is not, although he sees that the goblin runs off to attack someone. I think this is as odd as the dwarf not causing a reveal of the goblin in the diagram. I think it's best to avoid this kind of situation when designing quests.
Everytime I play
The Stone Hunter, I'm in doubt if I should place the next

if the player moves up to the

on the junction. I would say that the idea is to take out the first two, then the others. But maybe the idea is to be surprised by another

being placed when the player moves up to the first.