You do have a lot of questions and luckily i have all the answers
I guess there is an exploit in the game, how you can easily achieve families having 0 prestige or even below
Ofc there is , depends on the time in the campaign ,, the easiest is at start of the game by imprisoning the family members. Over time this will generate negative prestige and when it drops bellow 0 power base of that family becomes 0 . Even if loyalty is 0 character will not be able to generate civil war ..
Beside that I think reaching zero or negative prestige for families is hard to achieve (at least within normal gameplay - I hardly notice many situations where it drops and out of my head I don't even see what I need to exploit to make it happen)...does a family prestige of 0 really set power base of any character of that family to zero, meaning that even powerbase from other factors e.g. (like office, holdings, cohorts) is the ignored? Or does it only mean that the big chunk of extra powerbase for the family head is gone? While in the former case you are technically right then that powerbase is no longer factor (though I'm not sure if it makes the game better/more interesting), in the latter case I think this can also heavily backfire - in most cases not families are a problem at the same time and civil war is triggers when a certain percentage of overall powerbase is illoyal. If I now achieve to remove all family-based powerbase the pool dwindles a lot - and then I imagine that other characters suddenly become a problem, because the counterweight is missing (like governors)
Basically in this game to trigger the civil war you need family head/s to be disloyal ,, maybe not if your nation is small but proportionally to the size of the nation you need more powerfull characters to be disloyal to start the civil war . When nation size is big , one family head will not be enough, sometimes you need 3 disloyal ones to start a civil war.
Keeping family members imprisoned will generate negative prestige for that family and in some time it will become negative and power base of the family will be 0 .
Game can't make rebalance by making small characters enough to start a civil war , you need head of the family so if he haves negative prestige , power base is 0 ,, there is no civil wars and you can play with your own family only .
One modifier which you can use at start of new game too push family in negative is imprisoning and banishing their family members , each banish reduces i think 50 prestige, so , you will need 11 banishes to push family in negative prestige and / 0 power base,,, now im not sure do they keep generating negative prestige by being banished but i think they do if i remember correctly..
Yes it can backfire if you exploit civil wars to refresh loyalty of characters/provinces + to generate bonus PI , not having powerfull characters prevents you to have a civil war and since i know how to have 10 days civil wars i didnt use this mechanic a lot . I think maybe you could by making your pretenders unhappy to trigger the civil war..
Pool of characters is fine if you play with one family only,, cause they are greatfull family they multiply like mouses