I searched for this thread seeking clarity and have ended up watching a can of worms open, that said I have enjoyed the journey!
Pass Through Rock
This spell can be cast on any one Hero, including yourself. That Hero may then move through walls on his next move. He may move through as many walls as his dice roll allows. Caution! There are shaded areas on each Quest Map which indicate solid rock. If a Hero ends his move in one of these areas, he is trapped forever!
I think that the purpose of furniture in the game (apart from its undisputed aesthetic appeal) is to change the shape of a room that is otherwise fixed so “realism” aside I personally don’t allow Heroes to move through furniture in normal play as its purpose is to enhance gameplay by blocking movement. Walls also block movement.
I treat the name of the “Pass Through Rock” spell as thematic, much like the image on some spells and equipment cards, and use the spell description as my guide to its effects, so for me the spell effect is to allow you to pass through walls that usually block movement so I would allow you to also pass through furniture that would usually block movement.
Whilst I agree that wood is not rock, to my mind the HeroQuest world is based on elemental theory, everything is composed of varying proportions of the four elements so wood must have some amount of the earth element and it is the presence of that solid element that prevents a Hero from passing through it, if it was composed solely of air for example then the spell wouldn’t be particularly useful.
The more interesting challenge with this spell for me is around if this spell was originally intended to you to move into undiscovered areas, which the last clause suggests, then why doesn’t it clarify how the “reveal” rules are supposed to work in these circumstances.
If you say that under the influence of this spell the Hero can move through walls (possibly including furniture) but cannot move through unrevealed squares (which incidentally is the simplest resolution to this problem although not particularly satisfying) then it would not be possible for him to end his turn in solid rock and be trapped for ever so why mention an impossible option! The only reason I can think of, and it is tenuous, is that the warning is there to explain why you can't use this spell to travel through solid rock...because it could kill you.
So, if he can move to and through unrevealed/undiscovered squares as part of his movement under the influence of this spell then how do you best handle the revealing, after all and this has been pointed out already, he isn’t blind. As the spell doesn't mention the "reveal" rules, I would assume that they remain the same. As you declare that you are moving through a wall, you pause to allow the Evil Wizard player to place the room contents as usual, that way there is no need to allow or disallow the user to pass through monsters, normal rules apply.
Under normal conditions a Hero stood on a square adjacent to a closed door (a traditional entrance) may choose to open that door which reveals the room contents, even before he enters the room, or even if he decides to wander off in a different direction and never enter the room, presumably he pokes his head through the entrance and takes a gander.
Bearing this in mind I would suggest that a Hero under the influence of a pass-through rock spell must reveal the contents of a room or corridor by simply being in an adjacent square to it and declaring that he is passing through the wall as the wall that would normally prevent such an event from occurring is just another possible entrance for him, in the same way as an open door.
Unfortunately, this results in quite a spoiler event (especially if you combine a potion of speed and the swift wind spell with a good roll then you can reveal a fair proportion of the dungeon and ruin a quest) and potentially leaves monster marooned in a revealed/discovered room behind a closed door that they cannot open. This could be resolved by allowing monsters marooned in this way to open doors, but then they would have to be able to open all doors necessary to reach the Heroes, again a potential Quest spoiler.
One option to resolve this is to go with the Japanese rules and not allow movement through unrevealed/undiscovered squares and use that last clause to justify the restriction as below;
This spell can be cast on any one Hero, including yourself. That Hero may then move through walls and furniture on his next move. He may move through as many walls as his dice roll allows. He can cannot move into undiscovered squares including the shaded areas on each Quest Map which indicate solid rock. If a Hero was to end his move in one of these areas, he would be trapped forever!
Feel free to toggle the furniture piece on or off to suit your preferences
Interestingly this seems to be the interpretation used in the App.