1) I think 98 or something like that
2) as makaikhaan said, it depends, but I would say Byzantines in early period
3) It depends on your playing difficulty, but for (let's say) hard campaign, my guess would be around 1190. Although, I've seen extraordinary campaigns of fellow forum members who achieved 60 % victory around 1150-1160 ,IIRC
4) makaikhaan explained it all. Here's the campaign of my friend who wanted to achieve 100 % victory (and was extremely unlucky with rebellions): Losing 96 provinces in one year