I love this show and cast and crew so much:
Correct me if i'm mistaken - i haven't rewatched the older seasons so memory may be hazy.
What came first? The supernatural kids or the Upside Down?
I believe it was the kids ( for whatever reason they started to appear) and as we now know it basically Eleven that created Vecna when during their fight she tore open a gate to the Upside Down and pushed him through, where he was transformed into the creature we saw.
- Did the Upside Down always exist or was it created by Eleven and Vecna during/after their fight?
- Did the Upside Down creatures, the Demogorgon and the Mindflayer, always exist and only could enter our world once El and Vecna had their fight thus opening the portal or did Vecna create them? Alternatively all the creatures their are separate entities and do what they do on instinct or their own will.
- How did the kids came to be and how did the government notice them and get involved? It's obvious there's two factions within the government and one side would probably have no problem killing a bunch of kids in an assault on a private home.
Hard to say at this point. The Upside Down
appears to have existed in some form already but then One's arrival helped morphed it into what is now. With every answer we get, more questions arise. For instance, I don't accept the notion that Henry simply developed his powers on his own out of whole cloth and I firmly believe that something else caused them to manifest.
Before this season, we had been on the assumption that Eleven and the other children were experimented on by Brenner simply as part of the covert CIA operation that actually happened in our reality and that lead to the initial opening of the Upside Down.
But now we know that program (at least in Hawkins) was partially initiated by Brenner's desire to replicate Henry's powers which, again, I don't think is the beginning of the thread either. Now we also know the opening of the Upside Down by Eleven in particular happened sooner and drew Brenner's attention towards it sooner than we thought (the security footage must've shown the full extent of that fight so it's not just the cracks in the wall he gawked at). So I won't be surprised if we discover that the Upside Down has an even earlier origin.
Either i have forgotten some crucial details but these are the questions i would like answered before the show ends.
Pretty much everything you asked about is still up in the air. As I said before, we thought we knew the answers to some of these and it turned out we were completely wrong and I expect some of the answers we did get will also get upheaved in some way.
Also i would like to see what happened with all the other kids that escaped alongside El - if they don't appear on Stranger Things anymore might that be that they are "reserved" for the spin off show?
As far as we know (but, again, answers keep changing...), only Eight/Kali also escaped. As I posted earlier, her timeline isn't the clearest right now and I really hope either the upcoming two episodes or, more likely, the final season will clarify that point.
Come to think of it, Brenner chased after Eleven with a burning passion...but did he do the same for Eight? I know Eleven was his pet project and all (for more reasons than one) but surely he wouldn't want one of his experiments on the run. Unless Eight projected an image to all of them so believeable that she died instead of escaping?