I’m not sure I fully understand what you are trying to achieve? Are you trying to re-order the ‘fields’ that are returned?
That said, the reason your relevance returned “Singular expression refers to non-unique object” is that in its current form, you’ll have to pluralize ‘preceding texts’ and ‘firsts’ like so:
preceding texts of firsts "," of lines of files whose ...
( see Common Relevance Error Messages for useful information on diagnosing/addressing common relevance error messages )
You can also leverage the ‘it without a whose’ form (see https://developer.bigfix.com/relevance/tutorial.html) to maintain singular inspectors (and allow for singular treatment of items in a list):
(preceding text of first "," of it) of lines of files whose ...
This form would then allow you to manipulate each line separately, such as:
(preceding text of first "," of it & "||" & following text of last "," of it) of lines of files whose ...