I'm waiting for either a path forward to appear, or not. I was imagining the narrow ledge might open into another tunnel or something. I don't see jumping in the river as a viable or reasonable path forward. As far as I can tell, we're not in any real hurry. Our task here (as far as I remember/could tell from my previous skim-through) is to discover what's been disrupting the renovations of the temple on the surface, and put a stop to it if we can. It seems probable that whatever-it-is is also responsible for the sleeping sickness that's been plaguing the area, but I don't recall discovering any real evidence of such a connection. Which is to say, as a player, I see no reason to think that jumping in the river with nothing but a hope that it will take us somewhere useful (instead of drown us) is a good plan. And so, I can see no reason why Wallace would either.
I suspect that if we were so inclined, we could take the heads Sainrith has been collecting back to the sheriff (of Nottingham, teehee) as evidence of what's down there and request some additional assistance. At the least I would hope for some torches and lamp oil we could use to burn through the wall of thorns. Of course the smoke would be a concern in the enclosed underground environment. Doubly so with the nearby mysterious, giant, maybe poisonous mushrooms. Even better than that would be a squad of guards and hirelings to chop through and keep the area secure while we continue farther on.
So anyway, as long as we have a path forward that isn't obviously suicidal, I'm all for following it. And as long as we don't I'm all for going back and trying another way. I'm waiting to see which it is.