Yes, welcome Nightraven!!
I hadn't considered what Elena had in mind when she made the sculpture. My take is that maybe its significance was an unconscious act on her part. When Covenant thought it looked like Bannor, or one of the other Bloodguard, she said, "You tease me. I am not so poor a crafter." She didn't say anything about it being a combination, didn't try to explain it, or anything. She thought she sculpted Covenant. But somewhere deep down, she knew what she was doing.
As for how Mhoram figured it out from the sculpture, Bannor certainly helped him. The Bloodguard could very likely have understood why the New Lords couldn't figure out Kevin's Lore. They put no restrictions on themselves, knew that the Old Lords didn't either, but saw the New Lords doing so. If there were no other fundamental differences, they might have figured it out. And Bannor hinted at it as best he could, using the sculpture:
After Mhoram and the survivors of the Warward had returned to Revelstone from Garroting Deep, Bannr had explained the history of the bone sculpture.
In fact, he had explained it in unaccustomed detail. His habitual Bloodguard reticence had given way almost to prolixity; and the fullness of his description had provided Mhoram with a first hint of the fundamental alteration which had taken place in the Bloodguard. And in turn that description had led circuitously to the great change in Mhoram's own life.
So Mhoram's thinking, "Wow! What's so special about this sculpture that Bannor turns into a chatterbox?" And:
Intuitively, by steps which he could hardly articulate, Elena's marrowmeld sculpture had taught him the secret of the Ritual of Desecration.
In fact, Ahanna (remember the painter of
Lord Mhoram's Victory?) helped him understand it.
"...both ur-Lord Covenant the Unbeliever and Bannor of the Bloodguard require absolute answers to their own lives. With the Bloodguard it was their Vow. They demanded of themselves either pure, flawless service forever or no service at all. And the Unbeliever demands-"
"He demands," Quaan said sourly, "that his world is real and ours is not."
Another smile eased Mhoram's somberness, then faded. "This demand for absolute answers is dangerous. Kevin, too, required either victory or destruction."
So Mhoram understood that what Elena and Bannor were trying to tell him is that the Ritual of Desecration, and Kevin's Lore in general, could not work if your will was not absolute. And the Oath of Peace could not let them be absolute. You can't say, "I'm going to destroy my enemy. Well... You know... Maybe. If I can't figure something else out." War is war, and you go into it with the intent to win, or you
don't go into it with the intent to win. And apparently, Kevin's Lore is so infused with that attitude that even peaceful aspects of it are hard to manage with the Oath.
A thought just occured to me. The New Lords were certainly not pacifists. But I wonder if they actually would have been more successful with
some parts of Kevin's Lore if they
had been. I mean, Gandhi's brand of pacifism is absolute. Maybe the Lore would have responded to their absolute mindset, and they just wouldn't have bothered trying to use some parts of the Lore.
Regarding your first question, the one that's 2 fold:
1. Well, I guess just because he was brilliant. And maybe he saw - as a seer - certain things about Covenant and the white gold. Before he lost his prevision, of course.
2. We've debated this quite a bit here.

My opinion is that, though the Creator never explained how it could be, the wild magic is Covenant's. It's not just that he has the ring, because everyone seemed to understand that they couldn't just take the ring from him and use it - they needed Covenant to
give the ring - to give his consent. Elena was going to try to use the ring after Triock/
turiya took it from him while he was unconscious, but I believe she would have failed. He gave it to Troy, and Troy was able to use it. I won't give an example from the 2nd Chrons, since you don't say if you've read them, but it's the same idea.