I'm not sure what you are trying to say here, so I may be misinterpreting it - if so, I will apologize in advance.
I think it's important to make the distinction that the Kilrathi goals prior to the Battle of Earth are not really stated outright, but we can assume based on information of prior conquests and other tidbits we know of their society that they are typically more prone to subjugation than outright annihilation. They are basically aggressive expansionists, so they would take over worlds and then enslave the native populations. The decision to outright destroy the Human core worlds was out of the norm, but driven by multiple reasons.
As far as the Mandarins are concerned... I find it dubious that they could effect enough change from within in a meaningful period of time before the Kilrathi Empire was either defeated in another conflict, or destroyed by it's own infighting.
The fact is that without the false armistice, humanity had turned the tide and was in position to win the war outright. Narratively speaking, it makes for a more interesting story for Humanity to be on the ropes against an overwhelming foe, rather than slowly crushing a depleted enemy, and the armistice was the storytelling device that sets up the last ditch effort to pull victory from the jaws of defeat.
Thanks for your reply, and... sorry, I still need to work hard to improve my English.
Please let me put together my views into a list:
1. Because the length of the war between Terrans and Kilrathi was much longer than Kilrathis' expectations, the degree of hostility between the two sides may have reached a level beyond conventional. Therefore, if the Terrans surrender, the treatment may be even more terrible.
2. Because of the first point, it is uncertain whether the Terrans can maintain their own characteristics of civilization. In fact, I am pessimistic about this.
3. Kilrathi‘s own infighting may erupt between the Ki'ra clan and the Kiranka clan. According to the story, one of the major contradictions between them is the attitude towards the Terrans. If the Kiranka clan wins, in order to show its correctness, it is likely that the Terrans people will be sacrificed.
4. Even if the Ki’ra clan won, we are not sure if Jukaga could maintain respect for Terrans. At best, we can only hope that Jukaga would treat the Terrans like his father Vakka and tutor Harga did to those prisoners in the FAWCETT's world. Not to mention the complexity of managing billions of people. In this case, what would happen, for example, if other clans strongly requested some Terrans as sacrifice? I don't think Jukaga and the Ki'ra clan have enough prestige to suppress other Kilrathis' insistence on their tradition.
5. The best way to get a Kilrathi's approval, is to show your courage and strength. The surrendered Terrans will be despised by these aggressive Kirathi, and likely to become a kind of consumable in the war against Nephilim. How could a Kirathi follow the advice of a surrendered Terran to fight?
6. A friend of mine has said that the Kilrathi civilization needs a systematic and comprehensive failure. I agree with this view point. Even from their standpoint, perhaps such a failure makes progress in their society, though the cost is really high.