I haven't read most of the books, so some of my information might come from the wiki.
What Letho achieved:
1. Break of alliance with the sorcerers:
- What are sorcerers capable of? They seemed to play a big part in Nilf's defeat in the first War. In the first War for Upper Aedirn, Sabrina Glevissig more or less destroyed both armies with a single spell (a very powerful spell, nonetheless). So I am of the opinion that the sorcerers are capable of turning the tide of wars.
- "kings didn't need sorcerers support to oppose themselves while Nilfgaard attacked", I didn't read that part, but it was sort of by a twist of luck (an internal conflict) that triggered a chain reaction to Nilf's defeat. If the Nordlings had the backing of sorcerers, I imagine they would have an easier time defying Nilf's.
- After being massacred wholesale in Loc Muinne, the sorcerers might decide to just lay low, or they might decide to exact revenge. In the latter case, it would really hurt the Northern Kingdoms.
2. Death of kings:
- You speak pretty lightly of the death of the kings, as if finding an immediate replacement would restore order and all is well again. I would imagine that there would be utter chaos when the kings died and when the citizens hear about the impending invasion.
- Individually, the dead Kings, Foltest and Demawend are good strategists, invaluable in battle.
- More generally speaking, the death of Kings is a heavy hit on the morale of the army, esp. Foltest's, seeing that he was one of the heroic figures who led the Nordlings to victory against the Nilf. Morale matters a lot to an army.
- In terms of politics, there would be rebellious nobles who would probably withold their army or plot against the replacement (remember, it was the same kind of internal conflict that screwed up Nilf's plans). Even in the most ideal ending scenario, Radovid takes control of Temeria, Saskia takes control of Pontar Valley, there is still a lot of things to iron out. One cannot expect to assume command and to expect in the next second to command the army as if they were their limbs.
- In short, the whole Northern Kingdoms won't be functioning as a united front, at least not one that was as strong as that in the Second Wars.
3. Will the elves in Valley of Flowers flee to Saskia's state?
- I was under the impression that the elves under Enid are older, and they might choose to stay out of the war altogether for a few reasons:
a) Fear of Nilf's wrath.
b) They have peace and something closest to an independent state right now. At least, they're not getting persecuted (heck, all the kingdoms killed non-humans and incited pogroms because non-humans were suspected of colluding with the Nilfs)
c) They are not as bloodthirsty as the younger ones.
d) Given what the Nordlings have done in the past, they probably wouldn't so quick to decide and trust Saskia etc. It would take a long time to earn the elves' trust.
e) They would be betraying their brethrens in Valley of Flower if they went to Pontar Valley.
f) They have more to lose if they go to Pontar Valley. Think about this: currently they have a free state (not a perfect one), would they go fight and potentially lose all of that for some precarious freedom?
4. "So much trouble, so close to remaining kings"
a) It's not much trouble for Emhyr at all, he gets Letho started, and Letho just finishes up the rest while he sits in Nilfgaardian empire and prepares his army.
b) But you must be saying why would Letho go through so much trouble to defaming the sorcerers? You missed the point.
The whole point is to seed chaos and discord among the Nordlings (mentioned in Letho's speech), not to accrue the highest possible body count. c) Letho not killing the kings? If he had killed the kings, would he survive the scenario unscathed, he might get a few more but he would probably be overpowered by the soldiers sooner or later. But the most important thing is, that's not his main objective, killing the kings was just a means to his objective.
Not sure about Radovid. He might be next in the hit list if the Henselt assassination succeeds.
Hence I conclude that by employing Letho, the great Spice Merchant has effectively weakened the Nordlings while putting minimal effort.
2. I don't really know what his motives are, but slaying the kings is not "desperate", it's brilliant.
3. Probably how 227 described it, reducing the secondary characters to the figurative backseat. Unless they really want to start the game with a minimum of 4 initial states, I mean the engine might be super awesome at all, but I'm not sure if you can cover all the branches when you already have so many significantly different starting point.
That's all. Phew, I'm tired.