Is smite a Witchfire 40k?

A Grandmaster can only cast one power (or two with the strat), and Smite doesn’t benefit from the Warlord Trait because it’s not a witchfire.

Can you reroll perils of the warp?

If you roll either double 1’s or double 6’s with a Farseer, can you use their psychic test reroll ability/a command reroll to “save” yourself from Perils of the Warp? Yes. Perils doesn’t happen until all the re-rolls are finished and a final result locked in.

Can each Zoanthrope cast smite?

No, when you pick a zoan in the unit that’s only to measure range and where Perils originates from, not a single model in the unit cast the power, so unfortunately you cannot cast smite twice from a second model.