情态动词后为什么加现在完成时?who could never have imagined.

情态动词后为什么加现在完成时?who could never have imagined.
1.Fredierick William I,the king of prussia,who could never have imagined that his greatest gift to the russian people would have such an amazing history.
——————这一句中,为什么会出现could+现在完成时的结构,为什么要这么用?
2.另一个问题,The design of the room was in the fancy style popular in those days.
这个句子中popular充当什么成分,难道充当style的定语?它不是形容词吗?这是什么用法?
ps：例句都是英语课本上的,无误.
轩...2已经明白了。
（1）中，是说威廉在过去不会想到，完全可以用对过去的陈述语气啊，为什么用虚拟？