Just ran into these awkward situations during a Vassal game, and since i have just a clue on how it should work instead of a true sentence, i'm taking these questions to your attention:

1. If Greedo dies and takes a parting shot to the opponent's greedo, and he kills him, does the second greedo's parting shot have LOS through the squares the first greedo was at? Does the second greedo's parting shot happen before or after the first greedo being taken offboard?
2. If a hidden weequay shoots at Greedo and kills him, is he still hidden when greedo takes the parting shot, or is the hidden condition gone?
3. Does Opportunistic take place before or after Greedo's parting shot? (i remind someone talked about that in another topic and said before, but since i don't remember which topic was that, getting another confirmation is definitely useful)

1. A figure is defeated and removed from the map during step 7 of the attack. Both first Parting Shot and the other happen during step 7 of the previous attack, i.e. before the figures are defeated, so both figures will still be on the map.

2. Yes, Greedo is defeated during step 7, and hidden is discarded only after-the-attack-resolves (core/mission rule timing).

3. Opportunistic happens during step 7 of the attack when the target suffers damage. Being an attacker ability, it happens before Parting Shot (Parting Shot is triggered when a figure has suffered damage equal to Health, which cannot happen before the figure suffers damage). However, if it's the attacker's activation, the movement points go to the movement point pool. If it's not the attacker's activation, they must be spent immediately.

14 minutes ago, a1bert said:

Being an attacker ability, it happens before Parting Shot

Ain't the defeat and condition removal in this case part of their attacking ability? If it happens on the same step, shouldn't they apply 1st because of initiative?

They do not happen at the same time. Suffering damage happens during step 7 of the attack. After attack resolves happens after step 7.

After-attack-resolves abilities are resolved using the attack timing, not initiative order. (I think there's already a ruling and it will be in the next FAQ.)

There is:

Never thought about it, but your reasoning is totally legal. It means, that opportunistic is hard counter to greedo, because most of scum figures (except greedo and melee) can spend movement points and get out of LOS during step 7 of slow on the bonus draw attack against greedo before he even performs his... (as long as greedo survives the attack so parting shot won't trigger)