I know we have been told that the complete origin for this new Supergirl will be slowly revealed over years of stories. But we have seen some glimpses already of Kara's past, enough for me to try to put together a timeline of sorts. Part of the mystery here is that the glimpses of the past we have seen are from an indiscriminate time. So these are my guesses ...
My suspicion is the World Killer project was begun in Argo City on an intact Krypton. I doubt that the citizens of a displaced Argo would be interested in capturing embryos from other planets. They would probably be looking for a new world to inhabit. So let's start here.
So a couple of key things here. First off, my gut tells me that Zor-El was involved in the World Killer project. If I squint, the guy on the far right could be a young Zor-El. (Okay, I have to really squint.)
But more importantly, the 5th World Killer tube is shown to be empty here. So the fifth World Killer wasn't gestating with the rest of the brood. So at some point, that World Killer was added. It adds some fuel to the Supergirl being Number 5.
At some point after this, Jor-El figures out that Krypton is going to explode and tells his brother.
Zor-El decides he has to come up with some way of saving the city and comes up with the idea to use a force field. Now I am wondering if the force field ability is linked to the World Killers somehow. Maybe an innate ability in one of them that Zor exploited while they were in stasis. It would explain how NYC was encased. And why he 'designed' a force field but as machines that will provide atmosphere.
Obviously this is before Krypton exploded. Here Zor-El talks about Kara being put in a protective pod and sleeping through Krypton's destruction. So this would explain why Kara has no recollection of the destruction of the planet or knowledge of Argo's fate.
Given the context of the other stuff, Zor talking about his research sort of cements my belief that he worked on the World Killer project. But what does he need to tell Kara the truth about? What happened to ... what? Alura? Argo? The World Killers?
Or is it Kara herself. People thinking Kara is the 5th world killer could imagine him saying 'The truth about what happened to you. How I made you stronger.'
Unfortunately, Zor gets killed before he can let us know what he meant. He clearly recognizes the person who blasts him. Who would kill him? And why? Maybe he objected to what the World Killer project was all about and someone offed him before he could blow the whistle? Someone with a vendetta against the Els?
But remember, this was taped before Krypton exploded. So Zor didn't live long enough to see Argo survive.
My guess is that Kara is already slumbering in the pod when he records this.
But what if a distraught Alura, figuring she and Kara were the next targets, decided that the best way to save Kara was to make her stronger, subject her to the World Killer tech and hide her in the fifth tube?
Regardless, Kara is somewhere (pod or tube) when Krypton explodes. And someone is around to watch over Kara during the disaster. My assumption is it's Alura, even while she mourns Zor.
Argo initially survives, drifting in space, until the force field for some reason dissipates. Is this because the World Killers were awakening? And as a result, the control over their abilities began to weaken.
If the city is dying, whoever is caring for Kara would want to save her any way possible.
That someone somehow sets up the stargates that lead to Earth. Someone had to put them there. And that had to happen when Argo got trapped in the blue sun's gravity well ... so, recently. It would makes sense for that person to be Alura. She would know of Earth from Jor. She (at least prior incarnations of her) would know the science to create things. And who else would care enough to set up a star gate connecting to Earth.
At which point, the pod is fired. Given that the pod shouldn't be fired before the gates get put up, it makes me doubt this was an automatic launch. Someone launched the pod. Could it be Alura? That would mean she would have been alive recently? And did someone tinker with Supergirl's genes before putting her in the pod. I still doubt that she is the World Killer.
Anyways, my guess is we will continue to learn more and I will continue to amend my guesses.
Anyone out there with theories?