What's up with the Decrepit Village? Who lived there? When? Was it way back in 1372 when the drow fought with the Selunites? Was it more recent?
This is just the mining village for the gnomes. There are gnome bodies all over, and books scattered about that establish this. I want to say that there's a body you can use Speak w/Dead on, but I'll have to play back through to be sure.
Also, just to note, there's an interesting sign post somewhere along the path from the village to the myconids listing different areas along with updates about what happened to them.
I originally thought that the deep gnomes lived there, and the duergar attacked them and took them as slaves, or maybe the drow did. But now, with Grymforge, that is not true. The deep gnomes are from Baldur's Gate, come to Grymforge for a special mission. The duergar took them as slaves after they also came to Grymforge.
I believe the gnomes that weren't killed in the village were taken as slaves. Some taken to Moonrise and probably some to Grymforge, as well.
Notice, for instance, all the dead gnome slaves being kicked into the water. Any one of them could have been from the village.
The myconids obviously dwelled also along the shores, for when the duergar showed up to capture Thulla who was escaping from them, they killed a lot of myconids. Glut's circle was... um... where exactly?
Could have been anywhere, really. Personally, I imagine it was probably somewhere along the lake shore that we don't stop and actively look at. Somewhere we pass by in the boat ride. But really, I'm not sure much would be left after the circle was destroyed. Myconids don't seem to live in structures.
How long have those duergar been warring with the myconids and how long have they been in the area?
One of the books in the village talks about how a presence of Duergar is building up. My impression is that they destroyed Glut's circle before attacking the gnome village.
But who used to live in the village? I couldn't find anything that gave me any clues about this? Was it gnomes? Was it dwarves? Was it drow? Was it Selunites from 120 years earlier?
Gnomes, definitely. There are clues all over the place, dead bodies, books with information.
And how long has Blurg and Omeluum been with the myconids in the Underdark there? By the gods! They met Lenore and invited her to join the Society of Brilliance, but she hasn't been in that area for 11-12 years, and she was only there about 13-14 years ago in 1479. So have they been there in that particular Underdark area for 12+ years? Do they not go anywhere else ever? If they did, why didn't thy know that Lenore's been gone for 12 years?
Maybe they come and go.
Let's say I travel to Dallas every so often, and someone asks me a question, and I think, hmm, they could probably get an answer from this person I used to know who lives nearby in Ft. Worth, and then I direct them that way... it doesn't mean I'm actively up to date on what's going on in Ft. Worth.
In other words, they probably travel about. Knowing who Lenore is isn't the same thing as keeping tabs on her. They're not Lenore's keepers, after all.
Also, just a reminder, these are characters who come out of a 5e adventure module entitled Out of the Abyss. They're not stationary characters who only exist at the myconid colony.
And how the heck do Derryth, the dwarf lady, and her husband, get into the Underdark to pick mushrooms? We're having a hard time trying to find our way out, but they're just there, picking mushrooms, acting like they go there on a semi-regular basis to do so. Do they have some secret way in?
There are numerous ways into the Underdark that our characters don't know about. That's kind of a given. Why would we know every entrance and exit?
I also don't think everyone would be open to discuss their routes into and out of the dangerous location. It's like, "Hey merchant with valuable stuff, which way you headed after you leave?"
Another thing to point out: not all entrances and exits are available to the characters just because of the scale of the world. Your character, for example, can't go marching off to Thay. The location isn't built into the game, so there's no path for you to walk to get there. That doesn't mean no one can go to Thay.
So there are other entrances and exits. But not everyone is going to want to tell you about them, and sometimes they're not going to be available to your character because the game is only so big.
And why is there a Zhentarim stash in that area? How the flip do the Zhents get into that area?
There's an illusory wall in the Zhent hideout in Waukeen's rest. It leads to an elevator that goes enters the Underdark there. It's actually one of those entrances to the Underdark in the game.
Is it supposed to be from their secret hideout near Waukeen's Rest? If so, that's east of the Temple of Selune, but the Temple of Selune's Underdark entrance is in the southeast corner of the Underdark map. So wouldn't that mean that the Zhent hideout is further east and wouldn't be north of the Selune Temple?
I'm looking at the surface map and the Underdark map. I don't see an issue with the locations.
(bear in mind, there's some underground movement you have go through in both locations to hit the Underdark entraces, so I'm not sure how far off the surface locations that underground movement takes you, but...)
...in general, the goblin camp is south of Waukeen's Rest on the map, and likewise, the Underdark fort location is south of the Zhent stash.
And why are there minotaurs down there just roaming about? Are they purposely there to guard the Zhent stash?
Why are there monsters in the Underdark?
A couple of minotaurs live in the area. They survive and find food. I'm not sure how much more reason I need.
And why is the phase spider lair a way into this particular area of the Underdark when it is east, also, of the temple? Yet you can drop down from the Whispering Depths into the Underdark area of the map.
You're not factoring in the movement you take underground. As far as I can tell, you're just looking at the Blighted Village, calling it east, and then questioning the entrance point.
As opposed to figuring the Blighted Village as the entrance to the whispering depths and then consider the movement therein which leads to the entrance to the Underdark. And the same holds true of the temple entrance.
In other words, you can't just hold up the surface map over the Underdark map and punch holes through them with needles to try to line it up. There's another map in between.
And... um... the Festering Cove. It is UNDER Ebonlake and yet it has a big lake of its own, almost like an underground ocean,...with LIGHT at the end of the tunnel. A big gleaming sunlight on the horizon beyond. ???
The Festering Cove is not directly under Ebonlake. And yes, it does have a lake of its own. I wouldn't be at all surprised if the two water sources touched one another, or at least did once upon a time.
As for the lighting issues: there are lighting issues everywhere, including the owlbear's cave. This is a known issue all over the place; it's certainly not limited to the Underdark.
And lastly, was the Spectator turning drow into statues back in 1372 or is that drow party recent? According to one of the documents you find, it mentions statues near the Selunite Outpost, and that was back in 1372. However, the only statues we come across are present-day drow on patrol who got attacked by the Spectator. So which is it? Has the Spectator been around for 120 years, or is he a recent addition, and did he turn people into statues back 120 years ago AND presently?
I'm not sure what you're saying.
Yes, there are statues.
Yes, there is a spectator.
Yes, there is a book in the Selune Temple talking about the statues.
There is also a book in the Selune Temple talking about how the spectator couldn't have made the statues, so it's questioning what exactly did.
As far as the statues we find... what makes you think those are "present-day" drow? You're talking about the drow with the memory shard that's fading because so much time has passed? These are not modern day drow at all, not even close.
Nothing in the Underdark is making sense. Nothing.
I think the Underdark makes a lot more sense than you're giving it credit for.
Personally, I think there are some things that could be touched up, but I can't say I agree with your appraisal.