Well I can't remember which book, but a direct quote from Taox is "I'm a mother, we live long..." and Taox reappears to challenge them after Oryx and sisters become hive gods. This is after the mention of Oryx destroying multiple worlds. It is implied that Mothers are nearly immortal from the start, that is why Savathun desired to be one. She was considered to be unfit to rule the Krill or the Hive because she showed compassion through wanting to raise her own coven. The Awoken is pure speculation on my part, but to me it doesn't seem like coincidence that Mara Sov still looks like a teenager when she was part of the Twilight Gap, something like 40 years before our Guardian was resurrected. Eris lost her ghost in the Hellmouth and she spent years hiding in darkness from the Hive. So to me literally anything could have happened. When she used a Hive incantation to warp us out of Crotas throne realm she conveniently said it was one of Toland's tricks, but our ghost read Toland's journal and we heard no mention of Toland mastering Hive space magic. I'm thinking instead of Eris being a replacement she rather joined with Taox and became how she is now. Again, there's no proof to a lot of our theory (cooked up with another user in another topic) but I'm convinced.