Miss Waterford certainly felt that she could not do her subject justice on the curb of Jermyn Street, and so, like an artist, flung the bare fact at me and declared that she knew no details. 

I could not do her the injustice of supposing that so trifling a circumstance would have prevented her from giving them, but she was obstinate.

对这句话“I could not do her the injustice of supposing that so trifling a circumstance would have prevented her from giving them”不明白。

1）of supposing ... 这一结构 和 injustice 是“同位语”成分关系吗？

2）使用would have prevented 这里是虚拟语气，还表推测？ 搞不懂。就是这里为什么要使用would have done这一结构的含义？

3）还有这句话do her the injustice of supposing...这一搭配是什么意思？

麻烦老师详解，非常感谢！