Idk, I've always likened what I see in time travel to a tree. Merlin I think entwined the original trunk of timelines and they branch from there, specifically, when someone steps of the tracks of their fate and makes a free will choice. There are a few allusions to it, the mother's about new future's unfolding, Uriel and co enabling choice.
That's important, they're not preserving choice, they specifically enable it. Promoting new future's. Mmm, in a way, that makes the KotC vs Denarians battle about tilling the metaphorical soil and planting new seeds new possibilities.
I like the idea of a B2tF timeline where you have to go back to before the choice in order to effect it, taking Odin saying the effects of TT are twinned universes it makes sense combined with the multiversal precepts in place.
The current timeline very likely being one already having time travel effects.. but, it also seems to be very closed loopish, with things fluctuating in the story based on his memory of the event changing. That's an indicator to me to, if it's a past tense first person narrative then he has to be away of the changes of in some way have his memory buffered from it. The Occam's razor to me, is it's because he's directly involved in it in some way.
@griff, perhaps he wouldn't have to cut? I'm not on top of my weaving methods but I think crotchet for instance maybe? I was thinking, it's more like five already slightly different versions of Merlin went back on the timeline to some point before their divergence and tied them together, like crocheting a dreamcatcher, using tension from the different angles to give it structure.