although those elder gods are unnecessarily picky if they still want the stoner's blood after all the monsters paint the facility with the staff

After all, I didn't see a blue-collar worker on one of the collection stones, did you? :P

This was the first time in years that I was actually surprised by a surprise cameo, rather than some random cockbite writing a blog comment or INCLUDING IT IN THE REVIEW before the movie opens.

Also, I haven't seen anyone else ask this, but who was actually responsible for sabatoging the operation? I mean making it so the pot wasn't laced and the cave didn't blow. Was this kept ambiguous or did I just totally fall asleep when it was revealed? 

I'm pretty sure it's mentioned that Marty traded the laced pot for his regualar stuff, or that the fact that he smokes so much--or the specific weed he smokes--made him immune to Chem's attempts to control him.  I can't remember if this was actually in the movie or something I read someone speculating later, though, so take it with a grain of salt.

Also--the cave in  didn't go off because of a power surge, probably caused by Marty fucking around with the elevator controls.  "Upstairs" is the killing ground.

Sigourney Weaver's cameo was a complete surprise. And I loved it. When The Director comes over the PA system, I thought to myself "That sounds like Sigourney Weaver...I guess it's just a vocal cameo." And then she actually appeared and my jaw literally dropped.

Marty was immune to the toxins that they were using because of what he was smoking. My guess is, is the marijuana was just plain making him immune and not that he'd switched it with another batch or anything like that. I think the 'puppeteers' just didn't take that into account. I really don't remember anything being said about trading his stash, but I could be wrong. I'm pretty sure it was just the fact that he was smoking it that made him immune.