In my view, the confussion has to do with the difference between Indicative and Subjunctive modes that we have in Spanish, which tend to be merged as one in English. So:
"Acaso fue él quien lo hizo" and "Acaso sea él quien lo hizo" could be translated both into English exactly the same way: "Perhaps it was him who did it", but in Spanish there is a subtle difference. The former sentence should be used as a main idea, and it implies less of a doubt about who did it (in spite the "acaso"). The latter sentence would be slightly more open to a doubt and it should be used to complement a former, main, sentence.
I hope this helps.