The officer was the very Mr.Denny  concerning whose return from London Lydia came to inquire, and he bowed as they passed.

这个句子，只要把划线部分弄清楚了，一切就万事大吉。

concerning 介词用法，等于about。可以跟 inquire 搭配：inquire about/concerning。

whose return = Mr.Denny's return

把介词 conerning 提到关系代词 whose 之前。

The officer was the very Mr.Denny concerning whose return from London Lydia came to inquire,and he bowed as they passed.

【翻译】这位军官正是丹尼先生，丽迪雅上前来打听他从伦敦回来了没有，当他们走过时，他鞠了一躬。