I think the time frame is a few days, at most a week. The only evidence I really know of is the druid who mentions that they had been chanting for days, after you stop the ritual. I believe, in regards to Halsin, it went something along the lines that he went out a few days before you arrive but became MIA when the mercs returned. The xenophobia and poor leadership timeframe can only be guessed at, I don't think there are any hard facts given with that stuff. Though it is evident that Halsin was more interested in dealing with the Shadow curse rather than managing his grove. With what can be gathered it paints an image that Halsin is not a good leader. The fact that the shadow druids either swept in right under his nose or were able to get in and usurp control within days is not a good sign. Considering how quickly they got operation Thorn Shield going there was obviously a lot of festering resentment going on within the grove. With Kagha, I'm fairly certain she is supposed to be someone who was poorly trained, has a lot of resentment and was easily open to manipulation. Hence our ability to talk her out her decisions as easily as she seemed to have been talked into them*.
*Easy being relative to your luck with the gods of RNG.
The killing of the child comes across, and I'm pretty sure I've already stated it in this thread, more as a fuck up rather than a "muahahaa I'm so evil" vibe. If they were going for mustache evil than she would have simply ordered the snake to kill the kid. Not like Rath was actually going to do anything other than cry about it, its all dumb evil.