Situation:

[ ][ ][EXIT]

[Riot Trooper][C-3P0][Impassable]

[ ][ ][Luke]

Luke enters space with C-3P0 and then moves towards the EXIT.

1) Can Riot Trooper perform Parting Blow on Luke?

2a) If yes, what happens if Riot Trooper successfully stuns Luke (with Covering Fire). Won't Luke be stuck on the same space as C-3P0 if he no longer has any actions?

2b) If yes, does Luke receive the Distraction buff from C-3P0?

RRG page 15 "Interrupt" says: "A figure’s action cannot be interrupted while the figure is in a space containing another figure."

Thanks for the quick clarification!

Yes, you can Parting Blow while another figure is moving whether it is moving through a figure of not, because moving isn't an action that would be interrupted. (And Parting Blow is triggered by a figure exiting a space, it's not a during-an-activation ability.)

2a) If the figure is stunned while sharing a space, it is placed in the closest possible valid space chosen by the figure's controller.

2b) By RAW non-companion figures sharing a space are not adjacent to each other for other effects than the purposes of Blast and Cleave.

(Rulings exist, for example Mak can Disengage when the moving imperial figure is entering a space with another (non-companion) figure.)

Also see

https://community.fantasyflightgames.com/topic/271046-overrun-jet-trooper-and-parting-blow-stun/

https://community.fantasyflightgames.com/topic/239420-parting-blow-and-stun/