Is there a way to get both tribes at peace, or is that impossible?

Thanks.

I don't think so. You have to pick one or the other and I don't recall ever running into such an option. I guess someone could mod it so that you get the result you're asking about, but that's probably the only way.

^^^ No way at all. Unless someone made a mod to make it possible... which really is not a bad mini-mod idea.

I have tried unsuccessfully to be the peacemaker multiple times, and have covered all the possible scenarios:

1) You enter the One's camp first, they ask you to kill the Elders, and then if you do not kill the Elders, the next time you enter the One's camp - blam, Kobayashi Maru. (for them of course, hehheh)
2) You enter the Elder's camp first, they ask you to try and retrieve some genetic data on the One, and then you go to ask the One nicely and - blam.

The variable is set once you have interaction with the Elders. If you do not go DS on them, you cannot save the One.