If you're planning on implementing Lembas Bread from the beginning, you're going to have to make up advantages for each challenge. You can make the Lembas shareable, I guess.
Palantir could be 2-shot instead of 1-shot, it's really not too useful unless you're clueless/desperately need the info.
You could potentially make it so it reveals a single person's vote at another tribe's TC instead, that'd be much more interesting and fun to play around with.
Expiring at F6 sounds about right.
How big will the cast be?