Maybe I'm not seeing the big picture here, buy why is Terseon invading Redgorge again? Is it just to look for Alek? That seems a bit extreme, expecially since it's to the bad guys benefit if Alek is never found. Am I missing a vital plot point?
Vhalantru and the Cagewrights are attempting to get rid of all opposing organisations, forces and factors before they begin the ritual. The Chisel is one of these dangerous organisations who can begun to make moves within the city (ie the challenge against the government and specifically against Teresong) which can not be ignored.
Coupled with that is Nathatoran's age old lust for revenge on Redgorge and Sundabar's descendants which makes for an ideal opportunity to despose of powerful opposition and all reasonably legitimately and legally.
So using the pretex of the challenge and the 'threat' to the city by the Chisel, the Invasion of Redgorge is an excellent way to wipe out a major problem and prepare for the ritual.
Hope that helps
Delvesdeep