Even if that has a bearing on how time works in that particular instance, we still have the other time-travel event which happens in "real time" and - as I think on it now - is every bit as reliant on time travel not automatically creating a split timeline.
If our trip to three years ago only has us affecting a copied timeline and not the original, there is no possible way for Quickthinx be in possession of of Backrix's journal-from-the-future in the original timeline.
Therefore, as far as I can see it, if the rules of time travel are constant between the two stories (and I certainly hope that they are) then I think it is impossible for G'raha to have created a second timeline simply by arriving in Lakeland. Something had to happen after that point to cause the divergence, and I think it can't happen until he is interacting with history as he knows it, which only occurs once he is interfering with events in the Source.
To use the "three years ago" incident as an explanation point, my thinking is that time would only split if we did something that young Mide saw and couldn't possibly forget or misremember - but also something that present-day Mide knows didn't happen to her, and (unlike the reveal that the goblins were responsible for her friends' deaths) isn't just a reframing of the existing facts.
So if we'd acted to protect or heal her friends who got shot, that would be a change that made events play out differently to how present Mide remembers them because she saw her friends die.
On the other hand, if we were somehow able to rescue Dayan after young Mide lost sight of him but before he was actually pulled into the core, that would not have split the timeline. It would just mean that events turned out differently in that single version of the story and the one we saw never existed, because it doesn't have to.
If anything, I'd speculate it's the fact that G'raha comes from that other timeline that prevents it from being shut down as a once-possible future that didn't happen.
It was kind of funny the first time but calling him that every time just makes your argument harder for me to process.
I'm not understanding your point here. If Alexander got brought out of stasis in either timeline and travelled back to drop off Mide and Dayan in the far past, it would end up at a point before the timelines ever split in the first place.
Even if the timelines split apart as soon as G'raha arrived in the past, by Source time the absolute earliest point it happened was in 3.4 after Minfilia departed to the First. Alexander is going to have to go back a lot further than that.
You could handwave the genetic material thing as that somehow by random chance, their descendents managed to acquire the exact same configuration of genes... assuming that's even in play in this world and it's not some kind of weird aetherial equivalent.
As for the codex itself, I understood it that the concept of what was recorded in the codex might be cycling through the time loop, where ancestor-Mide creates the codex based on her past reading of it, but I don't think the physical object is the same one going around and around.