1) He's just a sick old grandpa and that was how he gets his entertainment is playing with and eventually killing the maids around him.
2) If you're talking about when Yan Xun and Chu Qiao are escaping and then get captured, it actually wasn't YWY who shoots them, but his grandfather's assistant who shoots the ice arrow to make people think it was YWY.
3) I don't think he realizes that she overheard them until the night that she takes revenge on the Third Branch grandfather.
4) Not sure who specifically poisoned the food, but I think it was plotted by Wei Shu You