I was musing about this for my latest playthrough. In my opinion, it would simply be best to admit that Aedirn and Temeria as we know them, are done for, and these kingdoms would in one way or another go to Henselt and Radovid, respectively.
• Adda lives: This gives Radovid a stronger tie to Temeria, which will influence things later on.
• Aryan lives: This prevents the La Valettes from making a pact with Nilfgaard.
• Roche's path (Act 2): This ensures that Saskia's rebellion fails; there are more important things happening, and the peasants and nobles alike need to be put back into order and unity (even if reluctantly). Stennis dies before any important events take place, helping Henselt's plans for Aedirn.
• Henselt lives: This allows for him to take over Upper Aedirn, and soon the rest of Aedirn. With Stennis dead, it's just a matter of time before this is so. This gives birth to a massively strong Kaedwen that is rich in resources both monetary and military.
• Roche's path (Act 3)/Neutral path (Act 3): This is where I am no longer entirely clear on how to handle the choices. If you take Roche's path and rescue Anais, she can on the one hand be given to Natalis, probably allowing for Temeria to be salvaged and stabilised. She is obviously not ready to take the throne, yet, but with Natalis in place until she is, the nobles and council of regents should be pacified and unified enough to get the kingdom back in shape. The other choice is to give Anais to Radovid, who in turn will immediately step in and make Temeria his protectorate; effectively wiping out Temeria as a kingdom of its own, sure, but expanding Redania immensely, and just as with Henselt and Aedirn, a massive super-power could be born, once again forging a strong bulwark against Nilfgaard. Now, there's a chance that Radovid can accomplish something like this, anyway, without Anais, if he is married to Adda, in which case giving Anais over to Natalis would still be a good choice if you want to build a strong north while still preserving Temeria to a large extent.
The real conundrum, however, is whether or not to take the Neutral path instead, going after Triss and foiling Nilfgaard's plan to start a witch-hunt, directly. This would leave Anais unable to help neither Redania nor Temeria, as she and Roche take to the road, but it would also prevent the effective destruction of the mages in the North. This was a goal of Nilfgaard's from the beginning, and while choosing (indirectly) to save the mages they would be able to offer their help in the upcoming war - and this is clearly beneficial to the Nordlings - the cost might be too high if Temeria is not able to be stabilised one way or another (by helping Natalis or Radovid). There is also the risk of Nilfgaard "offering sanctuary" to persecuted mages of the North, effectively tempting them to switch sides, as it were, and become an enemy of the Northern Kingdoms in service of the Empire, instead. This is pure speculation, of course, and seeing how Nilfgaard is generally really cruel to mages, it's possible this would never have been part of their strategy. Either way, while my gut tells me to help salvage Temeria (either via Natalis or Radovid) first, I am still a bit on the fence whether or not to put a wedge in Nilfgaard's plans, instead, by making sure the massacre of the mages never happens.
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