JeffC: From my understanding of how DLC works . . .
Bullshit. You're just parroting the company line. It's not your understanding. It's what EA/BioWare PR has put out there, except now it's coming out of your mouth instead of a press release.
The Prothean character has extensive commentary throughout the game, much of which puts the endgame 'choices' in context. His presence on the disc makes it clear that he was designed as a core character. Later, they created the Eden Prime mission as DLC and realized it was weak sauce by itself, so "bundled" the Prothean character with it.