First question. Its a prequel to Divine Divinity, so in fact even though its still in the divinity universe.. Its set even before the stor of the first game.
Second question. See it like this.. Because there are no set chains or anything, any quest will be a sidequest. Some will count towards the story, but even those are just as randomly encountered/found as other quests (reading dialogue and talking will help you find events/quests). The best answer would be yes.. There will be a massive amount of sidequests and the thing is you will probably do most of them while searching for the continuation of the story XD
Also no... Quests like kill X amount of this and get X amount of this are strictly forbidden. Those abbominations to rob people of their fun and time will not be included
all the quests feel smooth and even all "sidequests" feel like a storyquestline themselfs.
Third question. Kinda.. Henchman tend to talk to eachother or you sometimes, what they say depends on their attitude towards eachother (mostly how much you have used him as meatshield etc.)
However they dont have things like full background stories, personalities, real dialogue and quests attached to that UNLESS we reach the 800K goal.
Which MIGHT happen if it keeps this speed up till the end
This all is out of my head so people may correct me where needes