Here, this is what I mean as far as the map is concerned.
https://drive.google.com/file/d/1sdx8VQ27jk5-oXnFC5IQ1V9k-LIqUP5h/view?usp=sharingI could see that maybe the mountain pass will branch off, one way going to Baldur's and another going to the Cursed Lands. That's not really what we're told, but maybe that would explain it. After all, there are likely multiple roads to any given major city. Still, the Counselor for Ravengard tells you that the drow took the mountain pass to the Cursed Land towards Moonrise, but she makes it clear she and the Fist aren't going that way. She orders a Fist to go report to others, assuming its on the way back to Baldur's. So she makes a distinction between you going to take the pass and she and her Fist returning to the army.
And Barcus just walks off away from you at the windmill. So how the flip does he get down there to Grymforge? The Spider Lair? The Hag's Lair? The Zhent's Hideout? The Temple? Why was he in Moonhaven trying to get to the Underdark unless he knew of a way down there, but which of those ways makes the most sense as to how he got down there? Why can't WE take Barcus' way into the Underdark? Sounds a lot safer and easier than going through the Temple and a host of goblins.
And I literally gave you the definition of decrepit from the dictionary. "elderly and infirm or worn out or ruined because of age or neglect." It literally means that the village is old and run down. And just walk around it. It LOOKS old and run down. So, either the gnomes just got there and hadn't moved in and fixed up the place, or they weren't living there.
Ah, but here's another flaw I have with the decrepit village being the gnomes' home and lived in by the gnomes. Thulla tells you that the duergar "have them digging out an old ruin across the lake." So, um, what's the story? Were Thulla's people living in the village and then attacked by duergar just now, or were they enslaved and helping to dig out Grymforge and Thulla escaped from them by stealing the Boots of Speed from Gekh's sergeant?
One could argue that it's been some time since the village was attacked, and so the gnomes have been taken off to Grymforge for days now to work there. But that doesn't make sense either? Thulla's poisoned and badly hurt... right now as you meet her. It's been recent. And Spaw tells you that the reason the duergar attacked his people was to reclaim Thulla who escaped from them. A duergar on the shore says that they were hunting "a runaway slave." If she's a runaway slave, then that means she WAS a slave already, not that they captured the gnomes at the village and made them slaves and Thulla was the only one to escape. She was first made a slave and then escaped with the Boots of Speed. They were hunting her because she was a "runaway."
Why is Thulla so important? If they killed the myconids just to get Thulla, why'd they kill Glut's circle? I mean, they wiped OUT Glut's circle. It wasn't just a, "You're in our way and we want that gnome," kinda thing. They exterminated Glut's people. So where are all the myconid corpses?
And Gekh makes it clear he's hunting for Thulla. That's why he was there. He wasn't there to round up gnomes and ship them off. In fact, the dead gnomes at Grymforge imply that the battle occurred there first between the gnomes and duergar, not the village.
So this is how I'm reconciling this:
Thulla and Wulbren and clan went to Grymforge to make a fortune from adamantine. They started working there to get it working again. Then the Absolute sent Nere to hire the duergar and recruit them to unearth the Temple of Shar there. They warred with the gnomes and took the forge by force, putting the gnomes into slavery to help them dig out the temple. Little did they know, but some of the gnomes, including Wulbren, went across the lake to the Ebonlake Grotto area and the Decrepit Village. They hadn't been there for very long, but they were there because they were trying to dig up some additional precious stones. After all, one of the duergar corpses tells you that the gnomes were playing around in the rubble when you ask the corpse if he has anything of value. So he implies that the gnomes had something of value after digging in the rock piles there in the village. We know that Wulbren was in the village because one of the shacks has his little diary in it, and he was captured and taken to Grymforge.
So, it is likely that they first claimed Grymforge and sent some to the Ebonlake shore to mine there as well. Thulla escaped and tried to reach them to get their help to rescue the others at Grymforge, but Gekh's group of duergar and drow pursued. I'm guessing Glut's circle was somehow between Grymforge and the Decrepit Village, so when the duergar showed up at Glut's Grotto, he fought back against them, slowing them down. Glut called for Spaw and tried to get his help, but Spaw refused aid. Glut's circle was wiped out as a result, because they slowed down Gekh and his people, and then they continued on to the Decrepit Village before Thulla could rally the gnomes there to make a proper rescue attempt. There was a huge fight at the village, most were killed, and Thulla escaped to Spaw's Grotto. Gekh tried to pursue but ran into more myconids than his remaining units could handle. They were turned against one another or threw themselves off cliffs, etc., and Gekh retreated to the village. He and only a few remained on the shore while the rest took the surviving gnomes including Wulbren back to Grymforge. By the time you set out for Grymforge, reinforcements are on their way to back up Gekh, which is why you are met by duergar on your way to Grymforge.
Still doesn't explain how Barcus gets down there, or Derryth and Baelen, or Omeluum and Blurg, etc., but sure. I figure they could sail across Ebonlake also. That makes sense. But then, Grymforge must not be the ONLY way to the surface across Ebonlake. So... again... why aren't WE given any choice but to go to Grymforge? I mean, if I'm a selfish evil person just seeking power and I don't care about the Absolute, or I think I might fare better in Baldur's as opposed to risking going to Grymforge, if I knew there was a perhaps safer way out that several others have taken including a middle-aged dwarf female and her husband who's a befuddled moron, I might consider taking the safer route out of the Underdark. And if Barcus can get in from the same region we did without going through the Temple or the Hag's Lair or the Zhent's Lair or the Spider's Lair... why couldn't we go down the same way he did? We saved his life. Right? Wouldn't that be one of the first things we'd maybe say to him if we knew we were looking for a way into the Underdark to search for the Nightsong. Wouldn't we say, "Hey! Barcus. BTW. You're heading to the Underdark, are ya? Say... um... so are we. Got this nice quest we took called Search for the Nightsong. Mind if we tag along with ya? Come on. We just saved your butt. Whatdaya say? Forget the smokepowder satchel in your heavy backpack. Show us how to get into the Underdark. Eh?"