User blog comment:FairyRaven/Fairy Tail Chapter 410: Memento Mori, Review and Commentary/@comment-24490201-20141123170233/@comment-24490201-20141124173251

Well, maybe Jellal can fight. But I doubt Natsu or Gray would let him. They would want to defeat Mard Geer on their own. No, I don't think CS and OS will fight Mard Geer, even if they could (although, I don't really mind if they do). They will most probably help in defeating E.N.D.

The other theory you stated is plausible as well.