1) Can I beat them for good? I am raiding, pillaging and razing their sites fairly often. But not a single little pit of ~10 inhabitants finally disappear from the map. I have tried attacking their fortress in an open attack (Pillage) but there is a clown in there. And that guy managed to pick apart a fully steel-armored lord-skilled squad until my guys surrendered. Did you guys ever had some success in killing such a monster in fortress mode? And how do I destroy them for good?
I recall at least one report where giant animals have managed to slay demon in a raid. So breeding a small army of war jabberers supported by good tactician could work.
For taking out pits, there is the raze option.
3) My current mayor, then baroness, then countess and probably Queen in the future won't pick a spouse. She is befriended with a Carpenter but that's it. She does not have a single job right now. So she has plenty of Social time in the tavern. Do I really have to lock her with another guy in somewhere so she finally falls in love? I am talking about 7 years now in the fortress being a useless noble.
Socialization happens only with people one stands adjacant to, which typical player tavern designs don't really support, and slowly decays over time.
So pretty much, yeah (you could redesign your tavern but that'd still take longer); assuming she isn't asexual, unwilling to commit, or have personality traits or values preventing relationship progress.
6) I have a Library with 2 original books (so Artifacts) which I managed to buy from merchants. But my two scholars dont write anything, themselves. They are Scholars for at least three years now. They are Pondering a lot but nobody is writing any stuff. And they have PLENTY of writing material. I am talking about 14 emtpy scrolls and quires.
If you have artifact books, I don't think merchants brought them - visitors or raids may have though.
As for pondering, when I tested it took roughly half a decade of pondering from embark with proficient skill to result to write a book. You can set announcements.txt to display a popup announcement in that case.
If any books will do, picking someone who has churned out masterworks can work sometimes; they'll probably write about it. Or you can just steal books from someone else.