Further to @
fenriswolf456 's comments.
Kalad is a useful friend for them to have. What I do is having him being their wingman. For instance he and his paladin mates hold the portal open while the PCs go through, this helps me skip the boring Wraith fights in Umbraforge. He also heads up the investigations to guard and shut down that portals once the PCs report them. Generally he is their first port of call for passing on jobs they don't want to do. It is good, as it makes them feel like he is part of the team, despite being abscent.
Amyria is a more difficult one. How do the PCs have any attachment to her if she has no definable personality or history? What I did is make her a Deva that comes in the form of a 14 year old girl. I made her a pacifist cleric who powers all of her heals from self-sacrifice. So she is very effective at what she does, but the PCs are very protective of her and don't want her to heal too much as it hurts her. This stops them from trying to bring her on missions with them too often.
As for the "big reveal" in the Mottled Tower, the reason they end up starting the adventure is that Megan from the Farstriders has discovered a terrible secret and assassins are trying to kill her before she can tell anyone. It turns out that <drumroll> Sharshan is the bad guy! Oh shock, oh horror. Any competent group has reported him to the authorities half a tier ago.
I'm afraid that I can't help much with advice on the skill challenges. My group really has little interest in social interactions with NPCs, which is a shame. Things like that never go over well. I would suggest maybe adding in some encounters with assassins half way through the negotiations or what have you though. Maybe a seperate skill challenge to track down the assassins, doing so helps the main skill challenge.