I though in Mixed Mode (aka. "Don't play track one" format), the data track needed to be first.
CD-Extra/Cd-Enhanced uses multisession, since audio players only play the first (Audio) session, and multisession computer drives (all but a few obsolete 1x and 2x models) should access the second (data) session.
The audio tracks (of CD Extra) can normally be played by a CD-Player program on a computer, but if there's space, it's handy to put them in the data session as well, using whatever compression you prefer - or plain WAV files if there's room!