I'm a little unclear on the plan, which might mean that we're imagining two different things. I've certainly seen it happen. The Watchtower is possible, though not easy, to assault, and doing so is unlikely to alert the Gatehouse. However, having a fog cloud roll in right at the time that the wagon is passing by might be viewed as suspicious, unless you've got some great ideas about how to stage it, so having "Team Caves" just jump out and leave might be difficult - though it seems like no one is paying particularly acute attention to the wagon as it rolls by. Perhaps if you put the fog down on the ground, so that they only notice it if they happen to look straight down from the tower as the wagon rolls right through it... heck, I suppose that would be smart whether you assault the tower or simply have three PCs and Corrah jump out and leave. They don't have to go too far to the east to take cover amongst the avalanche rocks, and not far past that, they'll be "around the corner" and out of the tower's sight.
So I guess my question is: Fog Cloud, or not, Assault the Tower, or not, Team Caves leave on foot, or not?