Laura, I have a sneaky suspicion that the blue-furred elf is more than he appears to be. I don't think he is a spy for Galbatorix, though. Why? Because he was handpicked by Islanzadi for the job of protecting Eragon. Could he dupe his own queen? I don't know.
The fact Eragon thinks he's seen the blue-furred elf before, and he tells Eragon that it was his sister (or cousin?) he must have seen during the Dragon Ceremony is a hint. Perhaps the blue-furred elf holds knowledge of the location of the Well of Souls? We never did get anymore information on that in book three.
I definitely think the blue-furred elf has a major roll to play in book four. Perhaps you are right in that he is a spy, but I just don't feel it.
My major reason for believing Saphira will mate with Galbatorix's black dragon is her time spent with Gladyr. She will need an older, wiser, and more experienced dragon to "be with." I don't think she'd have any patience for a hatchling. Unless we are given another dragon and rider in hiding, we have only the black dragon remaining that is older than Saphira.
I do have a burning curiosity as to the hoard of power Galbatorix is accessing through his stolen cache of Eldunari. They are not dragons of the flesh, but they are dragons. I wonder what they know that they've managed to keep secret from Galbatorix?