I agree it seems very plausible that Lahabrea is responsible for introducing tempering, but I don't think we'll be handling a split timeline again. The memory crystal has to somehow independently make its way from Pandæmonium to our present, and that can't happen if we only fix things in an alternate timeline.

(Well, technically we could fix things in timeline A while the people of timeline B (connected to ours) are doomed and send us the crystal distress signal, but that just makes things even more complicated.)

Hydaelyn's talk about time "converging" seems more like she's talking about the two elements of the time loop starting in Elpis have met up. Our paths "converged" back then, we passed the information to Venat, and we went our separate ways – her through the slow path of millennia and us travelling back to where we began. Those two threads join up again when we meet her in full knowledge of what happened back then.