If "no return" is the obvious worst option -- being the dreaded "well, looks like we doomed the Karak to being lost in the warp?" scenario -- and if "immediate return" and "hours" is how fast it takes for the Karak to return, then... why are those in the middle of the roll? Surely, it's better if the Karak returns quickly, right?
After all, although people said "Well, we can spend more time fortifying" -- let's keep in mind that an easier way to get time for that is to just spend several days fortifying before Mathilde goes and does her thing at the Waystone.
So what is bad about quick returns, and good about slow returns?
Is it due to the implied reaction speed of the occupiers? i.e. "Immediate return means they sense something funky going on, and so pour out ASAP to try and fix it" and "'Days' means they are disorganized and... No, I still don't know."
The suspense is killing me. And that's not even to say whatever else might come later. ><