It's worth noting the Vex work primarily in timelines rather than timetravel. Because of this there is theoretically as many versions of the Nexus as there are different timelines. What the Restorative Mind was doing was taking one of these alternate copies and moving it into our timeline. This can't work on Atheon because it holds the unique position of "Time's Conflux." Atheon was the single point where all points in time and all timelines met. This is both his greatest weakness as well as strength. It gives him unique control over reality within the Vault, but it also means only a single copy exists. There is not one Atheon per timelines, but a single Atheon who simultaneously occupies all timelines. So when we killed him he's dead for good. There is not alternate version for a Restorative Mind to work with.

On to different questions now. The Restorative Mind Grimoire card starts with a quote that begins: "The Restorative Minds are of the Weavers." This is the only time I see a plural use of the title. I never did assume Theosyion to be the only Restorative Mind but it makes me curious if they would be able to just send a different one or if Theosyion was specialized in so.e way. Second, and the one that bothers me the most is that the Restorative Minds are of the Weavers." And as I recall wasn't one of Crota's sisters in of a power using the same label. So I suppose the simplest starting place is: What are the Weavers?

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The vault of glass does not exist in any known space or time the restorative mind can only bring back someone by "correcting" the moment in time they died but atheon is in the vault of glass a place where there is no time so the restorative mind can't bring him back

I don't think Atheon is dead. Even though we killed him. I have absolutely nothing to prove that.