Meaning of the Delta Rune (The emblem)

In Undertale, the Delta Rune is a recurring design element, showing up multiple times throughout the game, often associated with the royal family. Gerson informs the player that it's about an angel prophesied to free the monsters from underground, and mentions that some monsters have taken a more cynical interpretation, saying that the angel will "free" them from the mortal plane. The two interpretations were presumed to be connected to the Pacifist and No Mercy routes instead.

However, for how much the Delta Rune shows up, the actual prophecy is never really relevant; nobody besides Gerson ever brings up the prophecy, and the fact your actions were potentially foretold never becomes relevant. You only ever find out about the Delta Rune in an optional conversation. The Delta Rune is irrelevant to the plot.

When I first played the game, I thought there was something weird about the prophecy; on the Pacifist route, Asriel is implied to be the angel, while on the No Mercy route, Chara is implied to be the angel. (The waterfall mentions the angel returning to the underground, so it can't be Frisk.) It never really made much sense to me that the angel would be two different people.

And then Deltarune came out, and naturally the now-titular Delta Rune becomes a lot more relevant. Not only does it continue to show up as a visual element, but now the monsters actually talk about the angel. However, Ralsei offers a completely different prophecy, where the angel is a villain to be stopped by previously mentioned heroes. However, in Hometown the angel seems to be an object of worship, which is not what'd you expect from the new prophecy, so it seems the monsters still interpret it as the old prophecy. Why would the Delta Rune have two completely separate and contradictory prophecies?

What's more, I suspect Toby had the second prophecy in mind when he was working on Undertale. We know he had at least a rough idea of Deltarune in mind when working on the first game. The three triangles make more sense as a representation of three heroes than as a representation of one species. I've already mentioned the Delta Rune's lack of plot importance. I suspect the Delta Rune was always intended to set up Deltarune.

So the Delta Rune has two seemingly unconnected prophecies, and it seems it was designed with both of them in mind. What does this mean? I have a few theories.

1: One or both of the prophecies is wrong and/or misinterpreted. Perhaps one species forgot the meaning over time, or someone is deliberately lying.

2: It's like Super Paper Mario, where there are two opposing prophecies and the main conflict of the game is determining the correct one.

3: The angel will save or has saved the monsters, but will try to kill the Darkners, necessitating the intervention of the Delta Warriors.

4: The angel will "free" the monsters twice; first the angel freed the monsters from the underground, which is why they're on the surface in Deltarune. However, the angel will then try to kill the monsters, which is when the Delta Warriors show up.

My favorite theory, however, is that the meaning of the "free" prophecy does not depend on the player's chosen route. It depends on the game. And in both games, the angel is Asriel.

Deltarune is split into separate days, which involve alternating between the light world, featuring mostly returning characters and seems to be very domestic, and the dark world, which features mostly new characters and is more fantasy themed. However, when Asriel returns at the end of the week, he brings about the "world's end" mentioned in Ralsei's story. The "prince of darkness" finally appears in the world of light, and the two stories are tied together for the final battle.

That's my theory, anyway.

Wow, I sure went on about all this, didn't I? Thank you to anyone who actually read all of this, and feel free to comment about anything I may have missed or any of your own theories.

...Come to think of it, where did the Delta Rune even come from in the first place?