Now then — whether the culprit will confess or not has become the burning question for the silent ML participants.
Literal
Now then, culprit [topic-は], confess [or-か] [not-否か] [subj-が], silent ML participants [gen-の] burning-brow [gen-の] concern-matter [become-となる].
さて 'now then / well' is the narrative scene-transition marker. ~か否か 'whether or not X' is a formal set phrase combining the question か + its negative 否か, used for binary alternatives. 焦眉の関心事 is a compound idiom: 焦眉 (literally 'burning eyebrows') + 関心事 'matter of concern' = 'the urgent, imminent concern,' drawn from the classical Chinese phrase 焦眉之急 'pressing matter like fire singeing one's eyebrows.' ML is the English/net abbreviation for 'mailing list.'