Yeah, if I understand correctly (from the mention of .epub/.mobi/Calibre) the goal here is to get something that looks good on an e-reader, and those tend to have very narrow screens, like, 40 columns or less (not in a fixed font, but still). Fixed-pitch credits would look awful, probably every single line would get either cut off or wrapped.

TBH I'm not sure the two-column credit rolls can be made to look good on something that narrow -- you'll notice if you make your browser window narrow enough that my conversion gives up and shoves the song lyrics down below the credits.