Can't discern how a set of clauses function In the following sentence:


  外交部发言人耿爽14日就两名中国公民在日本不幸遇害答记者问时表示，中国驻日本使馆已要求日方全力破案，缉拿并严惩凶手，还受害者及家属公道。“我们对两名中国公民遇害表示沉痛哀悼，对其家属表示诚挚慰问，对凶犯的残暴行径予以强烈谴责。”耿爽说。


If I understand it correctly, the main clause in the first sentece is from 中国驻日本使馆已要求日方全力破案, and 外交部发言人耿爽14日就两名中国公民在日本不幸遇害答记者问时表示 is a subordinate clause. And the subordinate clause says something like:


  When the ministry of foreign spokesman 耿爽 answered to the question by the reporter on 14th,


However, how does 就两名中国公民在日本不幸遇害 function here? Is it the reporter that asks about the murder crime so the sentence should be like:


  When he answered to the reporter who asked about the murder crime,


or is it just added by the author of the post, so the sentence should be:


  When he answered to the reporter regarding the murder crime,


? If so why is it not expressed to use some regarding... adverbs such as 关于 or 于?
外交部发言人耿爽14日就两名中国公民在日本不幸遇害答记者问时……


It is not easy to understand a long sentence.
But, we can simplify it.

外交部发言人耿爽 (title + name) can be simplified as 他, he.

The date, 14日, is not important here, so we omit it for simplification.

两名中国公民在日本不幸遇害 is an event, so we use "the event", 该事件.

We use "the/his/her/their question", 其问, to represent 记者问.

The sentence then becomes 


  他就该事件答其问时……


(Don't you think it is much easier to interpret it now?
I suggest you using this method next time, when you read a looooong sentence.)


  When he answered the question regarding the event, ......


As you mentioned, 就 means regarding, in regard to, with regard to, etc.

Can't we use to or for?
When he answered the question to the event, ......
When he answered the question for the event, ......



If we want to use another Chinese characters to replace 就, the most common one would be 对.

Consider the following two phrases/sentences.

就事论事。
对事不对人。

Here, 就 ＝ 对.

Because we are used to them, it is not good to swith them.
But, the meaning is the same, and we can accept it.

对事论事。
就事不就人。


  他 对 该事件答其问时……
  外交部发言人耿爽14日 对 两名中国公民在日本不幸遇害答记者问时……