Can you use a fate to reroll one dice when setting the werewolfs strength when you enter the Werewolf's Den? Same question for Dice with Death...can I use a fate to reroll one dice for deaths roll? I guess the last would question would be in the Pitfiends, can I use a fate reroll to determine how many pitfiends must be fought?

Short answer: Yes, yes and yes, you can.

Rules, Page 6:

Once per die roll, a player may pay one fate token (returning it to the
stockpile) to reroll one die he has just rolled under the following circumstances:

1. Rolling a die for his character’s movement.
2. Rolling a die to determine his character’s attack roll.
3. Rolling a die due to the instructions on a card or board
space.

If a player pays a fate token to reroll a die, he must accept the
new result; he may not pay another fate token to reroll the
same die again.