So I was thinking of changing the role of the Emissary (otherwise known as Modra) a bit so that he works for the BBEG of this campaign rather then for Sarshan. That way he's the person from the Sinruth letter as well as the one dealing with Tusk.
Below is an altered series of the background events from the first few adventures. I'd like it if anyone spots any problems with my alterations, to let me know so I can fix them
Modra's Dealings with the Orcs
For reasons of the BBEG's own, he got Modra to oversee the task of throwing Elsir Vale into despair and ruins. Two years before the adventure Modra went to Myrissa and had her convince Tusk to unite the orc and ogre tribes in order to take on Overlook. In return Myrissa would be given a very important position in his master's forces. She accepted the offer.
Over the couple of years Tusk manages to gain a sizable force. Forces from Umbraforge are then sent to him so he may attack Overlook.
Dealings with Sarshan
After Tusk has spent almost two years amassing his army, Modra goes to Sarshan and offers for her to serve Modra's master in a very important position. Sarshan demands to be taken to Modra's master. He finally relents and takes her. Sarshan accepts the offer and is given a position above that of Modra's.
In her new position she refuses to give Tusk his weapons or any of the dark creepers and tells Modra to find another avenue. However one of Sarshan's underlings, Iranda, hears this and offers Modra the weapons and dark creepers in return for riches and safety out of Umbraforge. Modra accepts the offer.
Dealings with the Gnomes
Modra went to a group of gnomes and made an offer to its leader. Take control of some of the goblin bands within Elsir Vale and Modra's master would reward him richly. The gnome leader accepted and began gaining control of some goblin bands through guile, trickery and force.
Dealings with Sinruth
Modra kept an eye on things for a while and when he saw that Sinruth was doing quite well, went to the gnome leader and his new band of goblins. Gave the gnome leader his reward which included the gnome leader taking a more important (although at this stage not too important) role in the BBEG's plans beyond the Elsir Vale (the PCs will probably never hear of this leader). Meanwhile Modra has some of the other gnomes take on various roles throughout the world, while some are left with the band.
Modra then goes to Sinruth and offers Sinruth the band if he'll swear fealty to Modra's master. In return Sinruth will be given control of Elsir Vale. As a reward for Sinruth accepting this offer, Modra tells him the location of Castle Rivenroar.
Eventually Modra asks Sinruth to steal some artifacts from the Great Hall in Brindol. He does this to escalate things between the goblins and the denizens of Elsir Vale and deliberately times it so things should come to a boil just as the orcs are attacking Overlook (so the dwarves can't help the Vale and the Vale can't help Overlook). To help the attack on Brindol, Modra has Tusk give him an ogre to join the ranks of Sinruth.
Umbraforge Altered
After Modra's failures with the goblins and orcs, he wants someone else to mess up big time and so seeks to sabotage Umbraforge which would be an even bigger failure then his own (although he wants to do this without getting caught). Which was clearly not a possibility based on the adventure-as-written.
----
The point to all this is to connect the three adventures more closely and to hint at the BBEG's presence much earlier in the game much earlier. If the players question the goblins/gnomes they might say something like "The Emissary brought the Ogre from beyond the mountains" or Sinruth might say as he dies "The Emissary promised me power! I swore the damn oath.. *cough cough* *dead*"
Come Umbraforge it'll (possibly) become obvious that Modra, while powerful, was a pretty low-rung player and that Sarshan was clearly above him and much more powerful. So that way the enemies the player's face escalate while still providing a connection between all of them.