If I use Tsareena V's "Queen's Orders" attack, can I choose to discard zero Benched Pokemon?
Yes, you can. The attack will then do 20 damage.
Source: Fusion Strike FAQ; TPCi Rules Team (2021-11-11)
If I use Tsareena V's "Queen's Orders" attack and discard a Stage 2 Pokemon from my Bench, does it count as three Pokemon including the Basic & Evolution cards underneath that also get discarded?
No, the Basic & Evolution cards are all part of a single Pokemon while in play.
Source: Fusion Strike FAQ; TPCi Rules Team (2021-11-11)