Years, for sure. Game 2 itself will span a couple of years.
You’re right that marrow has some value in making aetherial blood; it slipped my mind, but I should have mentioned it above in the spoilery bits. This was foreshadowed in Game 1:
Breden’s face is drawn. "Angels, Terret, you’ve seen what a Harrower leaves behind. Every bone cracked open, eyes scooped out…the sacrifice needs pain to work best, they say. I don’t believe poor Yed here went through half as much agony as any one of the helots who get Harrowed round the Hegemony every day."
But stem cells are not the reason, and marrow alone won’t do everything that aetherial blood can do, so mass “Marrowing” isn’t the silver bullet. Also, non-crippling marrow extraction isn’t a possibility in the gameworld. In our world, the hollow syringe needle wasn’t invented til the mid-19th century; it doesn’t exist in Karagon yet.
Jinny could be a helot name, but it doesn’t have to be. 
Good question. I haven’t written it yet, and probably won’t until G3.
Well, without going into too much detail: the Archon isn’t the reason Shayard is unfree, and the crisis of the Hegemony isn’t yet so desperate that it would surrender any part of Shayard, even if its Shayardene client ruler fails to quell a revolt.