No problem! That is the main reason I joined this site, to have discussions.
As far as Akka being the only one who could call upon the Deep/Darkness, the BoS seems to directly refute that. Throughout the first entries where the Worms communicate with Auryx and his sisters, they refer to themselves as We, which I take to imply they are either switching who speaks or are speaking together. The language of The Sword Logic entry says that every Worm “god” can commune with the deep: “We will not give you the Deep, King Auryx — that power is for us, your gods. But we will teach you to call upon that force with signs and rituals.”
Concerning Savathuns strength, she was one of the original three who took the worms. She has had millennia of killing her siblings and countless civilizations (many of whom put our golden age to shame) to gain power, not to mention the tribute chain Oryx devised. I do not think she needed to kill Oryx in Truth to become as powerful as she is.