I didn't notice anything wrong with your post, no. (If anything, could you use single line breaks in dialogue, rather than leaving all that empty space between lines? I find it rather hard to keep track of the text and tend to skip over lines if people do that. But I suppose that it's mostly a matter of preference, if anything.)

I suppose you could have Kaedan go investigate the monster, though I think it would be preferable if we didn't spawn even more branches to the story? (Jack's word will overrule mine in this regard. He's been allowing quite a bit of freedom to the players indeed, though.) I must also warn you that this is no ordinary monster you'd be dealing with, but one about which it isn't even known whether it can be killed at all. 
As for the pre-existing characters, then those who're heading to Zerul City have actually briefly met it (and might or might not have a slightly better overview of it than anyone else). It remains unknown whether they'd continue to pursue to end the Withering or take a detour to try and eliminate the allegedly unkillable "monster", though.
As for the Withering, then it's existence is pretty much universally known ... any other details they've learned on the way would probably be spared by them. I suppose you could have Kaedan meet them somewhere along the way, perhaps in Zerul City (if we can ever get ourselves over there) and decide then. Or go hunt the monster and encounter the main group afterwards or somewhere on the way...