I activate Withcraft discarding 1 Mythos or Trauma to force a player test his Willpower (placing a Witch if failed).
Than activate it again in the same turn with the same player.
Than activate it again in the same turn with a different player.
Can I do this?

There is no limitation note in the card text, so I assume yes - you can.

Also, discarding one Mythos or Trauma card seems optional to me and aimed to reduce some of your witch spawnings threat cost to 1.

MyNeighbourTrololo said:

Agreed.