It would need to be an exceptional Stealth roll, to beat my Eagle's +13-14 Perception + Awareness.
But I see your point.
Like I said, it can carry risks, but it also allows opportunities to the Bjornaer.
The answer very heavily is YSMV, I think. There's no real canonical example of how group Tunnels work. Hell, what if I had an Intangible Tunnel targeting Structure - does it mean I'm considering to be touching every single person with a single tunnel, or am I connecting to every person with a different tunnel. Are there any examples of how to handle multiple tunnels at once? Are you troubled by trying to figure out which to cast into?
Well according to the revised guidelines for room you would have two options- either there is an ongoing effect which targets whomever happens to be in the room at a given time (i.e. you would have a single tunnel "touching" anyone in the room, anchored to the room) or an effect which is ongoing to the people who were in the room at the time of the casting (i.e. a tunnel to each person who was in the room the moment you cast the spell). I would assume this also applies to structures and boundaries.
Right, so if you have multiple tunnels to different targets, do you need to concentrate on each of them, making umpteen rolls? Or is it just one effect and one roll? (Assuming conc duration of course.)
I would go with Diameter or Sun.
“but it would be a very high level Vim spell to be able to cast decent spells down it.”
But that is when you follow up and cast the Range T variant! Come on! 
Generally speaking if I have a group spell with concentration duration I can concentrate on the spell regardless of how many individuals are affected. Now directing spells through this might get difficult, I could even see requiring a finesse roll to make certain you "touch" the right target.