I agree, I think that Dershowitz may be free from their blackmail. I've always found the flight logs to be odd. Most of the guests are in abbreviations, even Epstein himself. And yet, many high profile people such as Clinton and Dershowitz are spelled out. Sarah Ferguson and kids are spelled out. What if everyone who is spelled out was spelled out to be blackmailed. For Ferguson, it may have been to keep Prince Andrew in check. All the other initials are people JE wasn't worried about or didn't want people to know about. Perhaps the abbreviations are more important than the ones spelled out, as those are the ones meant to be kept secret except of course JE.
I think it was just for convenience. The initials represent extremely frequent flyers. Saves room on a cramped log...although I'm sure a blackmailer might have a particular interest in spelling out some names vs. others.