That mystical 就 again
Relating to your first sentence: 韦斯莱家却充满了神奇和意外。厨房壁炉架上的那面镜子就把哈利吓了一跳。

This 就 is the 12th meaning of 就 as an adverb in 《现代汉语词典》:


  表示事实正是如此：那就是他的家 ㄧ这人就是他哥哥ㄧ幼儿园就在这个胡同里。


It's meaning  12e in 《现代汉语规范词典》


  表示事实如此。 这儿就是我的家 | 车站就前面。


It expresses that the facts or the situation are really like this.
It can be seen as an illustration of or a proof for the previous sentence.

Relating to your second sentence: 到了下星期三，韦斯莱夫人一大早就把他们叫醒了

This pattern of 就 is explained here: https://chinese.stackexchange.com/a/1906/540
I'm not going to copy the answer I gave there :-)