Yeah the title seemed to be passed down over generations, I wouldn't be surpries if Zinnia's grandmother was the current Lorekeeper in her younger days. But can you show me a quote or when she says that the previous Lorekeeper had a sudden death, becaue I don't really remember that and I can't find anything that suggests it in neither Zinnia or her grandmother's dialogue. To me it would be very odd if Aster was Zinnia's predecessor since Zinnia treats her like a child.
Well yeah it's not stated that the MC does not become a Lorekeeper, but it isn't exactly stated that they do either. The problem lies more with saying that the MC is a Draconid. Zinnia saying that she has one more mission to fulfill as the last Lorekeeper of the Draconids would refer to her being the end of the line in everyday speach and fulfilling the Draconids' prophercy. If she meant that you were a Draconid as well saying "latest", "current", or just saying "a" Lorekeeper of the Draconids would be much more proper.
Of course it is possible that the Draconids spread to other regions as well. But to say that the MC is a decendant of Draconids that moved to Olivine is a fan theory at best. For all we know Draconids only refers to the clan in Meteor Falls, since Wallace refers to himself as a Sootopolitan when testing if you're ready to enter Sky Pillar. The player is obviously the very best because of gameplay. That's true in all of the pokémon main series games, using that to determine that they are Draconids is reaching. I think the only time they have even been close to explain it is in BW where the MC is seen as some sort of spiritual successor of one of the two brothers depending on version, but them actually having any blood relation is not mentioned at all. (Also I severly doubt that Steven is a Draconid since his family doesn't seem to know anything about them at all). My point was that in your original post you stated that the MC is a Draconid when that's just a fan theory of yours.
A wise man once said that there are no coincidences in video games, and there are way to many coincidences in the game to make the PC anything other than a draconid. Let's list them shall we?
Coincidence #1: An empathic dragon seeks out the player character because its mate is in trouble (the Lati's) so that the PC can defend it from the villain team. The defended dragon then joins the PC's team without the PC needing to battle it first.
Coincidence #2: The elderly couple on top of Mt. Pyre (likely draconids themselves due to the draconid peoples heavy involvement with the holy sites of Hoenn) then entrusts a child with one of the two orbs after the other has been stolen. Something they have no logical reason to do.
Coincidence #3: The player character then fights and either defeats or tames what is essentially an eldritch abomination capable of wiping out all life in Hoenn if not the world when the villains awaken it (Kyogre/Groudon fight).
Coincidence #4: Zinnia strings along the Player Character for the entirety of the Delta Episode, often going out of her way to do so to ensure that the player will be present when she summons Rayquaza.
Coincidence #5: Zinnia teaches the PC about the actual history of Hoenn, and the Draconids role in said history. Something, again, she has no reason to do unless she was preparing you to take over her job as Lorekeeper once she was gone. A role that, mind you, is essentially a tribal shaman for the draconid people. A role that anyone with even passing knowledge of anthropology can tell you is rarely given to outsiders.
Zinnia went to Sky Pillar to die while saving Hoenn, and she was grooming the PC to be her replacement.
It may only be a theory until someone officially confirms it, but the evidence is to great for it to be a mere coincidence.