Your comments are correct. There are killer NPCs running around right at the moment. Some of these high level spellcasting NPCs have an idea about what might happen before it happens through divinations.
The Forbiddance was dispelled because Solom was scrying the audience chamber and saw the Matron give the drow hand signal to attack Edwin. Solom started scrying once you sent a messenger to retrieve his book. Solom's Secure Sanctum (tm) protects from scrying by anybody but Solom. Solom also started the fire and had a hand in supporting the animation of hordes of undead in the Dodrien crypts. Solom also holds Eilos' phylactery as you guessed. Solom did not want Quertus to get his hands on Edwin's gear. Quertus didn't notice that the dispel was before Edwin's tattoo vanished because Quertus was focusing on Edwin's auras and not on area spells when the dispel happened. Solom did not have anything to do with the Devil or the Demon or even the illusion of Dorina. Edwin's contingency teleport was set to fire as soon as he took any damage.
The Bhaal-spawn was a distraction to make you think. An encounter like that is appropriate in the Dale-lands. They did the obvious thing, recovered Randal Morn's body and sealed the tunnel to the Underdark. They declined to try and kill every underdark monster (which is impossible). If you unsealed the tunnel or teleported out, then they would have come down after you.
The Devil was a potentially game-ending hostile encounter worth lots of exp. If you accepted either deal, its game over as PC independence vanishes. So you did the right thing to avoid his deal. Part of the point was to demonstrate a contrast between a Lawful Evil enemy and a Chaotic Evil enemy.
Turning down the gargoyle (actually a Forgotten Realms variant gargoyle known as a Kir-Lanan) was a minor mistake, but not a big deal. He had information he could sell. He works, along with a large number of other Kir-Lanan, for a Shadow Dragon I mentioned earlier in some of the posts. The Shadow Dragon is one of your hidden enemies who is also an enemy of Kurgoth and Irae.
One of the issues the party is beginning to run into is that your overall party power level is roughly level 20 (due to large numbers of PCs, LA, and Vampiric minions), but you are not that powerful with regards to magic. Quertus and Kripp are the two most powerful spellcasters, and both are members of Level Adjustment races.
I did consider how high a Balor could throw a Goblin. I figured fifty feet would be reasonable. The ceiling is roughly 200 feet above the commoner level, so throwing wouldn't work. The Balor used Telekinesis (violent thrust), which would work to move goblins (up to 750 lbs) the necessary distance.
With regards to the sense of accomplishment, I agree that it is beginning to be harder to find in the last few encounters. On the other hand, you just completed a major quest (capturing Szith Morcane) that took a year. Plus characters have gone up a number of levels since we started playing, and everyone is about to go up another level as soon as the Balor situation is resolved one way or another.
I had not planned to send the Balor to Szith Morcane. But, when you requested negotiations with a minion of Kurgoth, I asked myself, who would Kurgoth send? The CE answer, if you want something done right, do it yourself or send someone you trust implicitly. So here are your negotiations.
Of the "four chapters" of this City of the Spider Queen campaign, chapter 1 is Szith Morcane. Chapter 2 is getting to Maerimydra. Chapter 3 is getting inside the castle. Chapter 4 is the castle and Irae. So we're almost finished with chapter 2, possibly even chapter 3.
I am posting somewhat faster right now than I have in the past.
Campaigns don't last forever. If it stops being fun, then we should stop playing.
I did offer to let someone else take over as GM if they are interested.