Is there another way in or out of that room? How big is the pipe feeding into the cistern - could the fisherman have gotten into the tunnel through the pipe, either by exploring, an accident, or being dragged by the spiders?
The spirit rat continues to explore the room (including the cistern and pipe, if possible) until the spell runs out, at which point Barkot fills in the rest of the party.
The rat scurries through the unobstructed part of the tunnel at top speed, but as it approaches the web, it slows to 10' per round to be sure not to miss anything that might be concealed within the webs.
GM, please let me know if I need to make a check. If you’d rather do it behind the screen, that’s fine, figure that Barkot would spend the +1 from guidance on the first check.