请各位老师看一下这两个句子是什么样的结构，是否是定语从句？关系词后面引导的为什么是个短语?以下两句摘自剑桥雅思。

1. Like many other substances regarding the origin of which there existed some obscurity and mystery , ambergris in former time possessed a value, and had properties attributed to it, more on account of the source from which it was drawn than from its inherent qualities .

这里的of which 是定语从句吗？如果是which 指代什么？存在mystery of origin这种关系吗？

2. As Drucker saw it, the age of mercantilism in which economics was the only force that to need to be considered was dead.

这里的that的先行词是force吗？为什么that后面不是一个句子？

请各位老师分析指教。谢谢！