So, how I'm going to handle the NPCs (besides Aureus) is that they will follow the PCs orders almost to the letter (and within reason). While both Gareth and Timmins have some experience in the past, they haven't been adventuring in a long time, so they will defer to the PCs if things get intense.
Note that while Meridith and Wieland are older than Caerth (*), they did grow up in the same area as the half-orc. In the past ,they likely didn't think much of him, but since he helped bring back Lord Pendour's body, they've come to respect him at least. Meridith will likely follow his lead while Wieland might object once in a while.
Quinten has a camaraderie with Quinn, so he'll look to the chain-wielding fighter for guidance.
Brutus is the one wild card. He is young, inexperienced and has a lust for fighting. He is only 16 years old, so your PCs shouldn't let him go in, axe swinging, without Quinn (or Caerth) there to back him up.
* I'm assuming. Meridith is 21 and Wieland is 27. I didn't see an age listed for Caerth on the Rogues Gallery, but I'm assuming he's not older than Wieland. I guess he could be Meridith's age or a little older.
I bought the PDF version of the module, so I can use the maps.
Each square is 5 feet. The outside stairs lead up to the second floor of the tower, which is the main entrance. The spiral stairs lead both up and down. Down is the way you have to go to get to the entrance to the cultists' temple.
Since Caerth got out without the rooks seeing or hearing him, you can make a plan of attack (and still get surprise). However, if your PCs debate too loudly, they might hear you. Plus, there might other rooks around that might discover your PCs.
And don't forget that when you first entered the tower, you did so through a damaged section of the tower to the floor above this one, because its door was initially barred from the inside. Your PCs climbed up with a rope and then explored the tower. (That third floor is where you fought the ochre jelly, and where the chapel, with the sanctuary spell, is located.)
That hole hasn't been filled in so you could use it again, but it was a tough climb because the stonework is unstable. I think Aureus manged to stabilize it enough, so it didn't collapse. but that was a while ago now. I can't remember the DC off the top of my head (there isn't a suggested Climb DC in the module), but I don't think it was higher than 20 without a rope (15 with a rope). I'll see if I can find it.
Regardless, your PCs have done the climb before so they get a +2 circumstance bonus if they do it again. The new NPCs with you don't, however.
You could choose to send some of the group up that way and then attack from the stairwell whiel the rest charge in through the doorway. Note that if the rooks manage to block some of the characters on the stairs, it is not an easy place to fight from. Anyone without at least 5 ranks in Balance is considered flat-footed while fighting on the stairs, and if the rooks have the higher ground, they get a +1 to attack.
Note that it would be easy to collapse the decaying stonework, which would make a lot of noise. Noise that might attract the attention of the rooks. A few would investigate, at least.