|Douglas Muir 406|
If your players have a brain, be prepared to have to adlib a LOT of the adventures.
I agree -- it's definitely possible for the PCs to derail things.
I haven't found Grumblejack to be that big a deal. He's a big help with the Captain, but by the time you get to Ballentyne he's not that big a deal -- a CR 3 ogre isn't adding much to a group of 4th level PCs, and while Ballentyne involves a lot of role-playing and social stuff he's not useful for anything but combat. YMMV.
Unfortunately the authors wrote the adventure with a set story, and things happen that make no sense.
I can't speak for any other DM, but IMC if you'd really come up with a brilliant plan to sneak your army into the Vale, I would let you get away with it. Of course, you'd have to face and deal with all those good guys at *some* point...