介词短语作主语还是倒装

请问老师，下面的表示地址的介词短语是倒装句吗？假如真的的主语是a large store of chemicals，但又为何用谓语was呢？separated by a百度竞价推广i-fireproof partition是分词短语修饰the burning building吗？

In the rear of the burning building, separated by a 百度竞价推广i-fireproofpartition, was a large store of chemicals owned by the JR Company, who formerlyoccupied the entire structure.

这是表示方位的介词短语作表语，表语前置，主语后移致使的完全倒装句。主语被过去分词短语修饰，过去分词短语中的名词又被定语从句修饰，致使主语非常长。使用倒装结构，让主语坐落于句末，这就是英语的尾重原则。