this us confusing... why go all the way for such a little thing

Because it's the Crown Prince, the future king/emperor. Attempting against his life is attempting against the whole royal family. And tye Empress is pulling the strings with the priest to make it an even bigger mess. What I don't understand is why they want to bring Shuli and Jeremy down?

Because that way the empress gets to control the household of Shuli and Jeremy, as the previous head appointed Shuli as his successor after his death and the empress didn't like that