Yeah, that plot to take over a trade post is pretty powerful, and the AI isn't very good about expanding in the first place. Only one family had more than one when I initially took them over, and since the only two more have been built (in Denmark, obviously I took those over too). Might be because I am the doge, but if I wasn't I'd just save money until I can buy mercs (and hopefully when they can't buy enough mercs to beat me, works better in MP because people actually spend money if they are human) and go around conquering all of them. (Of course, humans are more likely to be able to competently retaliate afterwards too.)
How are the vendettas between the families?
Random event chains. Sometimes the text is amusing, but I wish they were a bit more fleshed out. I should declare vendetta on your family after you had my last patrician assassinated, not when a random event about a party pops up.
You missed the large negative relationship modifier between everyone in each dynasty, so they're probably going to try and kill you(r family) more often once you are in a feud, along with all the other things the AI does. I agree that feuds just starting via random event isn't that interesting, I reckon having the option to start one when a plot against your family by another family or when war is declared would be a better way to handle it, but that has its own issues.