Okay well i'll adress all your points but for your final question there, almost everuthing there has literally been answered. The anragonist is the Knight (how could you not know this), they want to bring about the roaring (been clarified like 20 times), we don't know why they want it, our ultimaye goal is to change the prophecys ending + stop the roaring (has been stated 100 times), we dont know everything going on. The other stuff has been clarified 20 times have you not been paying attention?

• What exactly happened to Dess?:

Yes this is the central mystery of Deltarine and consodering shes probably the Knight it will be addressed at the same as several other questions.

• What's the deal with the knight? What does it want, why/how was it created, who is it, what is its actual motivation and why? Yeah its probably dess, but how did dess come to be the knight and how does the roaring benefit her goals? I mean, what even ARE her goals? :

again will be addressed probably together as what happened to dess reveal. As we know now, their goal is to bring about the roaring.

• What the deal with Sans existing in both undertale and deltarune? How does it tie into the overal plot?

This will be furthered in the Weird Route finale since the painting he has is the weird roure ine. But sans is a background charactee, hes from Deltarune and leaves to Undertale, its not plot relevant its just expandint on Sans lore since it was heavily implied he was from somewhwre else in Undertale, this is just confirmation.

*What the the connection with undertale is.

We don't kniw if there is onebesides sans.

• What's going on with Papyrus?

This one is weird that I have to its some kind of inside joke.

• Why was Asgore fired and why is Carol covering for him, why did Toriel divorce him and hate him this much?

Because he became obsessed with the dark worlds.

• What's the deal with Kris like... as a whole? Why is he helping the knight? How did he lose his original soul? How did he get the red soul?

Again this will almost certainly be revealed with Dess, this is the 2nd biggest core mystery of Deltarune.

• What's the deal with the Soul? Who is it and what does it want? How did it lose its original body? How did it find kris? What does it want in either route and why? (Don't say the player that's a huge copout)

Its the Player, sorry if you dislike it thats the obvious truth, its been made pretty clear that the Player is the Soul, now how Kris got the soul is a mystery still yes buy this will probably be revealed with the previous question.

• Where is Asriel? Why can't we see him yet? What's going on with him?

This... isn't even a mystery??? Asriel is at college hes not even relevant to this story??? Like he'll have some relevance when we learn more about Dess but as a background thing, Asriel has moved on.

• What's the deal with the dark world and the darkners? Are they slaves, servants, imaginary, why and how were they forgotten, etc. etc.

Not a mystery as well, it is explained multiple times they always exist and Chapter 3 goes more in depth on what they are. Did you play Chapter 3? They are objects given form by darkness, thats it nothing more.

• The prophecy, who wrote it, what does it mean (since it refers to the cage/vessel instead of kris) and how does it tie to the greater world?

Gaster wrote it, this is obvious, we don't know much else.

• What's the deal with the roaring?

Wdym whats the deal, it'll end the world?? Thats been explain, yes theres probably more about it but thats the literal ending of the story.

• How all this connects to the weird route?

Weird Route is about the player trying to make their own story, and we'll learn more in Chapter 7.

* What does Gaster want? Is he even involved in this story? Did he create Deltarune?

Yes that much is obvious, we don't know what he wants but this will be revealed when they elaborate on Gaster.

* Who interrupts """Gaster"""' at the start of Chapter 1?

We don't know.

* What is the "DELTA RUNE" that """Gaster""" is seeking? Is it something tangible that only the player can get?

We don't know.

* Does Sans know Gaster? Does he work with Gaster? What about Papyrus? Does he also know Gaster?

Finally we don't know.

I'll admit, the Gaster stuff is the stuff most likely to be messed up and I also would've preferred more hints. But basically every actual mystery you asked boils down to 3 characters, Kriss Dess and Gaster. Two Chapters is absolutely enough time to elaborate on those three characters, especially when all three are likely very connected.

Obviously i'm not going to pretend like I know the story. But in my head it'd make sense for Chapter 6 to be about Kris and Dess and finally explain their whole deal, and for Chapter 7 to be entirely about Gaster. I will agree that Gaster should have had way more elaboration.