In a 'real' game (which this practice feels a lot like:) I, personally, prefer to keep most of the OCC out of the RP thread. In a learning game keeping it there seems as appropriate, especially when it is about a specific in game issue. Continuing here, though.
Discern Realities is a game mechanism, it has mechanical effects. I am not sure (as in 'literally' not sure, and not stating this for the record) that the second answer is something I can actually use to further my mechanical (or fictional) choices.
As a mechanic it is speaking to the player not the character (same way it is asked by the player, not the character). Saying the character gets a sense of foreboding does not give the player a lot to work with.
This seems like something the GM would mention as part of the scene description, not an answer to DR. We had probably assumed that already, which was why we were rolling instead of just grabbing and turning the pedestal.
The question asked was 'What should I be on the lookout for?'. That usually has two types of answers: the positive ones where it reveals something that could be useful, though there is a question for that as well, so it is more about 'opportunities' than 'items'; and the --more common-- negative ones where it reveals some detail of what might happen next. The second often acts as a warning, saying 'don't do that', or 'if you do that, be ready to do this next'.
If a question does not make sense --maybe it does not apply ('who is in control?') or there is no way for the character's actions to lead to an answer-- the GM can always say "ask something else instead", and maybe give a suggestion.
"Nothing" is always a valid answer, though not always the most satisfying.
The answer is true, though, so a GM can not say "nothing" and have 'something' happen.
It can be challenging to find the right level of description. In live play a GM can notice the players' eyes going glassy as they lose interest, in PbP there is no feedback till after the description is posted. Feedback from the players is the only real way to get the level right, each group is different. I encourage players to speak up if they don't feel their questions have been answered (DR or otherwise), many feel shy about it and don't want to be seen as criticising the GM, so having the GM ask for this sort of feedback is essential.
The GM can always give more description if the players seem to be asking for it (or ask for it explicitly). Planning to give more does slow things down (waiting a few days to how they react), so, if one knows more will be required, giving it upfront is possibly better.
The players have no sensory stream other than what the GM tells them. Their characters of course can see everything in the scene, smell the rot, hear the screeches, feel the vibrations, all of which allow them to navigate the world and not make stupid decisions. But without the GM's descriptions the players are blind.
GM: In general, I would say you can go into a bit more detail than you have been. You are free to add things like 'as you approach the pedestal you get the feeling that ...'.
Sometimes a player will jump ahead to doing a thing when the GM expected them to roll DR first, this might appear as them handing you a golden opportunity, but make sure they they understood that they were doing so (my having Jon stick his head out of the illusion was exactly that, I tried to telegraph that there was no carefulness to that act, and gave others the chance to stop it; I would also have added an OOC note stating this fact if it had happened), possibly rolling back the fiction if there was a misunderstanding. Watching for these misunderstandings is a good way to learn that more description was called for.
I don't think DW is very explicit about this --other games tend to make it clearer. Only call for Moves when the outcome is interesting enough. If failure would not be interesting, don't ask the player to roll dice, just say 'yes' and move on.
When it comes to Discern Realities and Spout Lore, don't use them to reveal details about the world. If the players rolls a 6- you don't want them not getting those details... unless that lack of information would be interesting, in the short-term while players are still thinking about it.
