What I don't get is that caliborn claims the 8 kids show up to the final showdown through John's retcon powers, but if that is the case then it shouldn't already be part of the existing timeline since it doesn't adhere to stable time loops. do you know what's going on there? Or know where it's hinted about?

But it does adhere to stable time loops, as a result of the fight Caliborn’s juju is sent to the furthest ring so that John could find it and get his retcon powers to take the kids to the fight or the existence of Lil Cal.  Both the pre-retcon and post-retcon timelines are needed in the Alpha timeline.