David_Jay,
re: "...I can assure you that as many times that I've heard that "heart of the earth" is supposed to be some Jewish idiom, it is not."
Actually, the idiom/figure of speech/colloquialism is referring to the 3 days and 3 nights and not to the 'heart of the earth".
re: "The colloquial speech works with Semitic dialogue about counting days."
But was it common for Semitic dialogue to say that a daytime or a night time would be involved with an event when no part of the daytime or no part of the night time could occur?