Can you play Roxie to discard Pokemon from your hand if you have no cards in your deck?
No, you cannot.
Source: TPCi Rules Team (2021-02-04)
When I use Roxie to discard two Pokemon that each have the "Blow-Away Bomb" Ability, do those abilities stack so I can place two damage counters on each of my opponent's Pokemon?
Yes, each of the Pokemon gets to use their own "Blow-Away Bomb" Ability when discarded by Roxie.
Source: Cosmic Eclipse FAQ (2019-10-18), TPCi Rules Team (2019-10-31)