Is Korrok responsible for all the weird shit that happens to them? Including the soy sauce and the parallel universe they stumble into at the end and the worm creatures like Shitload?
The sauce? Maybe. The parallely universe? Yes, but actually in reverse: Korrok is a consequence of that universe. The worm creatures? Yes, totally.
Why doesn't Korrok just kill Dave and John or erase them from history now that they've breached the universe?
Because he had bigger plans. Bigger plans that would have killed John and Dave anyway. If you mean after they come back... one assumes that the explosion had an effect on Korrok too, despite the Eldritch Abomination overtones. A better question would be why the Shadow Men don't just erase Dave and John instead.
Possible reason: the Shadow Men are from the future (hinted at by how they are some sort of time travelers, as they can erase someone entirely from the past)— in which case if they erase too much from the past they risk erasing their own existence or even their own universe entire.
Dave and John are special in some way, which is the reason they survived taking the sauce. Presumably, they can't be directly rewritten like that.
It's implied also that They specifically have plans to use Monster Dave to further their goals. Possibly they need John for this too. Since they proved to Monster Dave that they can erase Amy from history any time they want, they more or less have him by the balls.
How did Robert Marley get his hands on the soy sauce?
I'm guessing the same way as everyone else- somebody dealt him it and somebody dealt them it and so on. Where it originated is a different question.
It's said that Robert once lived in the Mall where one of the Ghost Doors to the Interdimensional Bazaar is located. It's possible that the people behind that place were the people supplying him in hopes of spreading the Sauce.
The idea is that the sauce does what it wants. It has no rules, he just found it one day.
What was the deal with the the robotic Amy Dave briefly sees?
I thought that was just supposed to be a replacement- a pretty crappy attempt to cover up Amy being taken.
Also, it's more a mannequin than a robot. But that makes the hole "bag full of fat" deal even weirder. Did they just expect the people on this side to not notice if they sent a bag with her exact weight in her place?
I was under the impression that the mannequin was a failed attempt to switch Amy with monster Amy. Like they tried to beam monster Amy in but it didn't work, and all that they managed to get across were some unassembled raw materials (the mannequin and the bag of fat)
Yea, I suspected that but... they had already sent two monster people into our world. Why were they still experimenting? From the moment they got Monster Dave in, they were already proficient in doing that.
The bag of fat (at least) was from the intermediary world, the pocket dimension which is and isn't part of the homeworld of John, Dave, and Amy. Until the very last example ( Monster Amy, who was only in our world for a few moments and might have been about to experience snapback anyway), all of the so-called Monster People were "special cases".
It might be a sort of Equivalent Exchange situation. In order to take something from this world, they have to send something of equal mass in its place.
Why didn't Monster Dave go evil?
Maybe something to do with that spider eating scene. Or Molly.
What's a different reality and what isn't? The door they go through at the mall with the elevator and Robert North, is that an alternate reality? North says he's from there and he clearly wasn't from Earth, so I thought it was. But then they go through the portal and it's said to be the first time anybody has ever travelled between worlds/dimensions/realities. So just what the hell was that in-between stage?
The "interdimensional bazaar" at the mall seems to be more like... a "ghost location". Something like the chamber under Robert Marley's trailer. It's just an "overseas operation" from Shit Narnia into our world, but still part of our reality.
In a deleted scene in the movie Robert North explains that very thing: If the two worlds are like two items of clothing woven from the same thread, the cave portal place thingy is the thread.
Why does no one even bother learning Robert Marley's real name(yea, me included)? Even the cop seems to just use it once when telling Dave who was that on the photo and then never more.
If first person shooter games were only retconned into existence a couple of months ago, how come John and the sportscaster were both familiar with their tropes during much earlier events? Granted, you can get around that by simply concluding that Arnie was delusional, but that seems a bit of a cop-out.
I'm unsure of the "earlier events" you're referring to but I'm sure they can edit something into history the same way they can edit something out of it.
Same reason no one else noticed. Or, for a better example, same reason Dave was able to remember that guy's wife who got Ret Gone'ed, but not Tony. It seems to go off Doctor Who rules for Ret Gone: If you're special (you've used the sauce or are a time traveler), you can spot the changes—unless its a major part of your life, in which case you can't.
They weren't. Arnie is remembering an alternate reality in which they were, just like how the Pope died in the assassination attempt.
This is explained in the book. When the Shadow Men erase something from history, they erase it in the past too. Like it never existed. However, people's memories don't always change 100%. Sometimes they can remember flashes of the way things used to be, or they get an odd feeling that things were different, kind of like Déjà vu. However, this is rare. It happens to few people, and even in those cases, the change merely overcomes their memories slowly. Even John and Dave experience this. Something is changed toward the end of the second book... to how it's been in the first book and the rest of the sequel. And they didn't realize it.
This can also be explained by Arnie being dead. This is implied by Dave noting that Arnie saw the wig monster in the crate usually fast. Something about Arnie being dead allows him to remember some of the changes and to see the wig monster more easily then most.
Book 2 shows that changes made by the Shadow Men update almost instantly even if it should result in a paradox. They erase Amy's hand resulting in Amy having the ghost hand that allowed them to reach that point in the first place. Too many changes happening at once means that some people will remember things one way while everyone else will think it's always been that way.
In the book, someone sends Dave Scooby-Doo glasses, and it later turns out Amy can see all the weird stuff (including her phantom hand) through them. In the movie, Dave just randomly gives her the glasses to see the ghost door. Where did the glasses come from and how did he know they would work?