1 The entrance hall was so big you could have fit the whole of the Dursleys’ house in it.

这个句子so big后面省略了as if.从句谓语could have fit为虚拟语气。如你所说，如果改为现在时态则为：

The entrance hall is so big （as if )you could fit the whole of the Dursleys’ house in it.

2 Mr. Brunner had this look that wouldn't let you go— intense brown eyes that could've been a thousand years old and had seen everything.

这个句子的intense brown eyes是look的同位语，其后的定语从句第一个谓语could've been是虚拟语气，但第二个谓语had seen everything是陈述语气的过去完成时，表示过去的过去。如果这个句子改为一般现在时态，则为下句：

Mr. Brunner has this look that will not let you go— intense brown eyes that could be a thousand years old and has seen everything. that从句的第二个谓语用陈述语气。

3 另外如果想表达她哭的声音足以把房子震塌应该怎样使用虚拟

She cried so loudly as if it would have the house collapse.