Ah, I misunderstood. I wasn't quite sure if you meant weeks/month in story-related time or actually not being ready to post until then.
Keystonian incursions into Zeal won't happen until much later down the road. Essentially, that's the second phase of the project. The subplots will more or less end and Keystone's attempt to return fire begins.
The idea of Sigma (and possibly Gil, depending on how Translanka wants to work this out) being "officially" returned to Keystone wouldn't be a simple release. After Argus flees Zeal, the Council is going to be pissed - after all, one of the brightest students that they chose to become a member of the Temporal School just decides to jump ship and screw up their plans - and then successfully escapes. So they don't look incompetent, they'll dispatch Nanashi to track him down and brutally put and end to his life; however, they're curious fellows. The Council is going to want to know why Argus did what he did and learn as much about him as possible so that they don't make the mistake selecting someone similiar in the future. For that task, they'll ask/demand Sessimine to hunt Argus down and throughly dissect his mind before the Nanashi wipe him out.
Conveniently, there also happens to be someone detained in a previous excursion in stasis - who is believed to be a member of some underground resistance movement and who has demonstrated an unusual resistance to magic for a simple Keystonian - from the very era Sessimine will be heading to. Sigma could be dragged along for the purposes of trying to get more information out of her and so forth.