1. At some point the murderers turned to Mary, pointing a pistol at her heavily pregnant belly.

解析：这是一个含有现在分词短语的复合句。

主句是：At some point the murderers turned to Mary，现在分词短语是：pointing a pistol at her heavily pregnant belly。

在主句中at some point是做时间状语。现在分词短语是做伴随状况状语。

2. Instead of being reduced to a weeping wreck, Mary was strangely calm.

解析：这个句子是个简单句。

其中的instead of being reduced to a weeping wreck是做状语，其中的being reduced to a weeping wreck是动名词短语作介词的宾语。

在动名词短语中，weeping是现在分词作前置定语修饰名词wreck。

3. On June 19th, at Edinburgh Castle, Mary gave birth to the boy who would become James VI of Scotland.

解析：这是一个含有定语从句的复合句。

主句是：On June 19th, at Edinburgh Castle, Mary gave birth to the boy，定于从句中从句是：who would become James VI of Scotland。

在主句中On June 19th做时间状语，at Edinburgh Castle做地点状语，主语当时间状语和地点状语同时位于句首时，时间状语在前，地点状语在后。

在定于从句中，引导出的定于从句的关系代词是who，先行词是boy。先行词在定于从句中做主语，是指人，所以关系代词who和that都可以使用。由于是做主语，不可省略这个关系代词。

4. On hearing the news, Elizabeth's reaction was to cry, “Alack, the Queen of Scots is lighter of a bonny son and I am but of barren stock.”

解析：这个句子中的on hearing something这个结构表示“一听到......马上就.......”，做时间状语。