@DK9027 There is a possibility that something goes wrong for some reason because of some quality of the other dimension, but it seems unlikely that something as drastic as the physics being different would result in a King's Canyon that is that similar to ours. Even if the physics are different and the Revenant body is trapped in the other dimension, it still doesn't prevent a new Revenant from rising again.
The IMC could maybe restart the project, but I doubt they could do it, since the Revenant facilities are heavily guarded, and they've been stopped before by these same guards.
Wraith is the only character capable of traveling between dimensions, but keep in mind that Revenant does have experimental technology, plus there could be a Wraith in the other dimension that is under their control. (He probably hasn't invaded other dimensions because it's too difficult.)
Who knows about the artifact? It probably has something to do with Revenant, due to the color scheme, the nature of how experimental it is, and because it is in the other dimension controlled by Revenant.
The Syndicate would definitely keep tabs on the legends, but it'd be foolish to keep them under 24/7 surveillance. Especially with Crypto, the Syndicate doesn't know everything and it would be difficult for them to learn. They couldn't and probably don't keep every legend under 24/7 surveillance. (The legends are basically celebrities too, so that'd be a bad PR move.)
While it is true that most of the legends don't have a good enough motive to betray the group, it seems like something that will be elaborated upon in the future, plus we also have Bangalore, who really despises Loba. The time the letter was sent/delivered doesn't matter much if a legend did it, since most of them would probably remember.
But still, why would an outside group inform Revenant, the scary immortal homicidal robot, about the legends planning something in secret that could be a threat to him?
Also, even if Octane slips some information, he definitely wouldn't slip enough for Yoko or whoever she works for to send the letter to Revenant.