Couple questions and am I missing something - minor spoiler ?

I'm all ears as far as what you consider a "closed timeline" to be. I've never heard of that expression before.

Nor do I see the relevance to Novikov. Novikov's principle only prevents self-contradictory timelines from existing. But of the many possible self-consistent timelines, it doesn't hold one of them to be a "forgone conclusion" at all. They're all possible.

Yes, when a self-consistent timeline is selected, the events of that possible future have already happened, but nothing dictates that timeline to be selected. As the billiard ball scenario shows, there are an infinite number of solutions.

A closed timeline is the frame of the loop. It has manufactured itself and in need of the course correction to do so. Therefore any deviation from that is impossible. That's the gist of the principle and accepts that the outcome is a foregone conclusion. According to his theory, paradoxes cannot exist. It's ad hoc and not very accepted because of that. I'll point out again, it was just his way of sidestepping the grandfather paradox.

A ball moving forward is knocked toward a time warp pocket by that ball from the future is just another way of saying the ball entering the pocket was inevitable. Can't happen without alt-verses because something had to create the loop to begin with.

What does work is a ball is traveling toward a time warp and goes into it without intervention, comes out back in time and strikes the ball before it enters the loop, knocking the initial ball out of the timeline creating a second one. However, that glance directs the ball from the future in timeline A back into the time warp. Thus it keeps going, while the original ball is off in another timeline. That works if Blaine is the original ball. It's not an event out of nothing, which is what Novikov runs on, which is just a way of saying it happened because it happened. Wizards and magnets and *beep*

His principle is incomplete and shallow. That's why Caltech and various other theorists came up with other ones like the second one I described.

Multiverses are another deal. Unless Blair Witch is an Otherverse from BWP, which I guess could explain why so many things are different...like Heather suddenly having a brother she never mentioned.

Ah.... you mean a "closed timelike curve" or "CTC". https://en.wikipedia.org/wiki/Closed_timelike_curve

Now, I know you like to address what really might happen, but in fiction, Novikov's principle is use many times. Just a few:

o "Terminator": John Connor exists because he sent his father back in time to impregnate his mother.
o "All You Zombies"/"Predestination (movie)" has Jane existing because she impregnated herself!"
o "Time Lapse": Finn's paintings only exist because he sees them in photos sent into the past so he can paint them.

It's called a "Bootstrap Paradox", and Novikov's principle merely attempt to refine the concept as to why some paradoxes work and others don't. That first link on Causal Loops I posted goes into the bootstrap paradox and Novikov.

And since BW is also fiction, there's no reason the events couldn't have also been written as a bootstrap paradox. The see a tape of them in the forest, which gets them to enter the forest and make the tape.

Now, since Godel showed CTCs are mathematically possible, we'll have to see if similar phenomena can happen in reality. I understand you have your objections, but I don't believe it's been proven (or disproven) one way or the other.

If your point is things don't have to make sense in fiction, you could have just said that and I'd have agreed. Since we're in a thread about things that don't make sense, other speculative fiction is hardly support for a claim.

To put a point on it, since we are viewing the tapes, it prevents the loop. There IS no closed timeline. An alternate timeline is the only explanation for the tapes existing outside of the loop for the sheriff or whoever to find. I'll readily accept this if there was something in the movie to suggest it...but there isn't really. It's a "wouldn't it be cool" thing. It works if the troupe we see are stuck in sort of a chronological snow globe the outside world is looking at.

Novikov doesn't attempt to redefine paradoxes, it suggests they're impossible. IE, you can't change the future because the future is set. Things happen because they can't NOT happen. Lost kinda ran on this. There will always be some intervention to prevent the paradox.

I understand you have your objections, but I don't believe it's been proven (or disproven) one way or the other.

I see no evidence in BW to suggest alternate timelines. Seems like straight-forward fixed timeline fare with a bootstrap paradox setting things off. Novikov's principle specifically allows such paradoxes.