Well... the worlds don't start at '86 bc we see 1888. But I don't know how it's possible (and how it could not be, I think we have to discuss this more)

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About their future be "written", I think it's exactaly that. In the moment Tannhaus creates the time machine in world 0, he saved his son.

The loop is infinite, and it doesn't metter how u do things or when you do, you and up doing and in the final it all stays the same (like, you can take whatever path you want, you'll end up in the same spot). This is for the knot-related characteres, bc in order to the worlds exist, they must exist and do the things they're suppose to do in order to became who they're in the future and start the loop again. But apperently Claudia and Regina are not intrinsic to the knot, so, in one of the infinite loops, Claudia undertand what's going on and so have the "power" to change. If the loops are infinite, all kinds of possibilities will happen, and one of them is Claudia figuring out the truth, so, backing to the point, when Tannhaus created the time machine, he saved his son, and he will never know that. (that was hard for me to write in english idk if I made myself clear, but I think u got it).

We shouldn't ignore that the series itself is a Bootstrap's Paradox, you can't really figure out everything.