Why is 就 needed in 美國國務院稱華府仍未就卡舒吉案下定論? If it's needed, shouldn't it precede 仍未?
For question 1

The word 就 actually bears a lot of meaning, you can check it out here.

Commonly, we use 就 as would, for example,

 你   再    吵  ,  你  就    死     定    了.   
You again noisy, you would die for sure.    (Literal translation)


But, in formal literature (such as news), 就 is normally treated as a preposition as with regard to. To be simple, you can think of 就 as 對 in such situation.

For question 2

In modern Chinese, adverb comes before preposition.

adverb -&gt; preposition


Since, 就 is a preposition in such situation, and 仍未 is an adverb. It is normal that 仍未 should come before 就. 

P/S: You won't want to use 就 as with regard to when speaking, people won't understand it.