Makeinu: One nice thing about Paranormal games is the flavor is very consistent. You can tell what faction did a kill by how the player died. (Vigs and PGOs use guns, dopp kills leave a bloody mess and little to no body, and aliens use energy weapons and leave half-melted corpses.) Also, if you're blocked by a Psychic Warden, the flavor will mention chains- things like that.
Now, with a player missing, you are correct that it's either an Xenozoologist or a Harvester. If Caz comes back tomorrow, it's a Mind Stealer. If not, either a Xenozoologist or an Organ Harvester. We won't know that distinction until D3. The Xenozoologist is more of an Outsider win condition, since it doesn't care about dopps or town winning. That said, it's typically considered a bit pro-town since it wants to leave with two townies, a dopp, and an alien, which are typically anti-town- trading two townies for two anti-town players is a good trade.
The Mind Stealer is oddball and I don't think we've had one. The Organ Harvester is clearly anti-town (and anti-dopp.) Based on the low player count, I'd say the Xenozoologist is unlikely (but that's no guarantee.)
Oh wait, there's one more option- an Exterminator with Combat Camo. I THINK the player is found dead the next day in that case.
As far as no kill, there are a few options:
1. The dopp team targeted Caz. When a player is abducted, all other actions fail on them.
2. The dopp that used the kill was blocked.
3. The dopp target was either protected or hit an alien shield.
4. The dopp that used the kill was redirected (Mind Control Ray) back to another dopp- IIRC that causes them to not kill. Or maybe that's only if redirected to self.
5. The dopp team chose not to kill.
The top three options are the most likely.
I'm trying to suss the different factions, and it's easier to ask. Town (human), Aliens, Doppelgangers, I assume are all separate factions. Do the Aliens and the Doppelgangers normally work together, or in opposition, i.e., both are scum (I assume), but are they one faction or two?
If they're one, and we know from the only night "kill" that it was an Alien, that means Aliens replace humans as well? If they're two factions, then we're possibly not down three Town from start, we're down more, which tightens the odds a bit. Independents always do. Three Doppelgangers and #? Aliens?
We know three crew were replaced, from the OP. We don't know if one of those replaces was an Alien, or I don't anyway, or if there's a fourth or fifth crew "missing" as well, only we don't know because they weren't body replaced as the Doppelgangers do it.
Town and Dopps are just town and scum. Each Alien has a different goal, but none of them are directly aligned with the dopp team. (Survivors typically favor the scum team, since a win with them is faster.)
By the pod count, we know the scum team has three original members. Aliens do not get counted by this, nor do Rogue Dopps. All aliens have some sort of tech that makes them appear human during the day (this is flavor only.)
Does this clear things up?
(PS I know there's no content in this post)