I tried to establish a timeline of the movie to get what was going on. It's still full of holes, things I can't explain but I think it kinda holds.
Why were there two Hilliers?
The first Hilliard who traveled to the past (from the original Timeline I'll call Timeline 0) made himself immortal in Timeline 1 (the timeline created when they made the first change by traveling back in time). So he kept existing past the time when he was born in Timeline 1, which resulted in 2 Hilliards:
Immortal Hilliard (the one who had traveled in time and was 2000+ years old)
and Other Hilliard (who was born in 1947 of the affected Timeline 1)
I didn't quite get what the conspiracy was.
Well, the Lunologists didn't want anyone interfering with their plans to time travel and keep "fixing" things so they were covering it up. Also, the US government was mixed in since they were the ones the "tunnel" under the bayou belonged to (I think).
All this created an atmosphere of conspiracy and cover-up that Immortal Hilliard used to get to Other Hilliard.
What was the function of the church?
That's a little more dicey to me. In the first timeline, I imagine Hilliard created the Church to recruit people who were open to the possibilities of time travel and affecting the world through change, to later establish a colony on the Moon, Lunopolis.
In subsequent timelines, it was Immortal Hilliard who published the Holy Lunar Testament in 1940 (before Other Hilliard was actually born) in an effort to expose the Time travelers who were living in Lunopolis, and the changes they were making.
It failed because Other Hilliard and the Moon People just used it to create the Church by cashing in on the people who believed it and making cash from it.
What were the two of them seeking to accomplish?
It's unclear but I think they both started trying to make the world a "better place", which Immortal Hilliard realized was impossible because if you keep making changes, you don't know what you are affecting.
What was the significance of The Event?
This one is a bit of a pickle. The Event appears to be a shift in timeline. It's a little complex and it really depends on the theory of time travel the movie is subscribing to but the moment Immortal Hilliard kills Other Hilliard in that timeline, Other Hilliard cannot time travel to the past and change the timeline
Up to that point, the movie seemed to operate on an Alternate Histories theory of time travel, where the time travel creates another timeline/dimension.
But right then, since Other Hilliard's death creates a timeline shift, that must mean that the past of Timeline 1 (the documentary people timeline) has already been time traveled into and changed.
And that's also how Immortal Hilliard's disappearance can be explained but it creates problem of coherence within the movie.
How did it signify the end of the world?
It did not. The only times it's being referred to that way are during the broadcast, by a journalist saying some people are interpreting this at a "end of the world event" and it's most likely because of the date, which was the last date of the Mayan calendar. A lot of people thought it was some kind of prophetic calendar and that the date would be the end of the world.
In the movie, Immortal Hilliard gives us an explanation for this. He implies the Mayans were Moon people who had traveled back and says that "nobody has ever gone past that date".
Of course he only knows it from his point of view. There is no history after that date, as recorded by the Moon people because they never get past that date. However, the world keeps existing since the video is leaked to Youtube and broadcasted.
What happened after the event when the scene became a shopping mall?
It's the timeline shift. The event created a paradox that prevents Hilliard from traveling back in time and becoming immortal. So a lot of things that were changed in the past, are not. For some reason that turns this restaurant into a store in a shopping mall
Can someone please give their interpretation of the movie?
I reconstructed a timeline but I'm not 100% sure about it because some things don't make sense, as it seems the movie operates simultaneously on two contradictory time travel theories: Alternate Histories/Timelines and Mutable Timeline.
However, to me, the bigger picture is that there are just powers humankind is not supposed to wield. Changing reality because you are trying to fix mistakes ends up creating more trouble, more *beep* for more people and the only way to go is to face your reality and try to change the future, not the past.
I think that's what "the test" Hilliard mentions was all about. Maybe a more advanced alien race or time traveling group is testing Hilliard and the others to see if they get that. Of course they don't and they fail every time.
For every lie I unlearn I learn something new - Ani Difranco