Ok so, this posthumously born Princess of France (Let's call her Catherine) would probably be given some sort of courtesy title and land in France, due to being the eldest and only child of the previous King. Mary Stuart now has an heir for Scotland too, so that's good. If she decides to return to Scotland in this situation, I can see her having to fight to keep her daughter, perhaps resulting in a later journey home and slight dissolutionment with France. If everything goes OTL in Scotland and she has to escape, I can see her returning to France, especially if she thinks he can get her daughter a suitable husband as a French Princess there. From there I see her having the Bothwell marriage dissolved and remarrying someone of standing in France. She could then do the same for her child. Maybe her Guise family would send a small force to try and regain Scotland but ultimately she and her daughter would live out their lives in France.