Abilities » Farewell Letter
Can I use Unown's "Farewell Letter" Ability to discard it from my bench if there are no cards left in my deck to draw from?
No, you cannot. The order of operations for Farewell Letter should be to draw a card first, then discard Unown. If you are unable to draw a card, then you cannot discard the Unown.
Source: TPCi Rules Team (2019-03-28)
Abilities » Farewell Letter
Can I play Unown onto my bench, activate its "Farewell Letter" ability to discard it, and then use Revive to put it back onto the bench and use "Farewell Letter" again?
Yes, you can.
Source: TPCi Rules Team (2015-12-03)