Honestly, the only thing I don't get is, how having Fran, a defected Viera, "Guide" us through the Golmore Territory causes the resident Viera to NOT attack us.

What with the whole idea being that leaving the Forest is frowned upon, returning to the Forest is unacceptable and bringing outsiders with you is just unthinkable...

Sure, they allow people to leave the Forest in order to deal with a threat to the Forest, but dealing with Ultima was not the reason why Fran left (It's the reason she agreed to go back and help us, but it's not until we highlight that Ultima exists and is a threat does she even know about it)