Well, I guess they just went with the simplest explanations for everything here.
Fairy Tail has two teams just cause they had two make it. Jellal is just pretending to be Mystrogan and that's okay because "SHHHHH don't tell anyone!". And Mavis can apparently just hang out whenever even though she's dead.
Mirajane! Erza!
Damn choosing is hard.
(lol @ Elfman finally getting his manliness zapped by none other than his sister)
If it's not Isuzu-chan Mii~
"He doesn't seem to be evil... But more importantly: Is he strong?" Fairy Tail is still faithfully following the first master's philosophy.
It really shouldn't be much of a problem to ponder if two Fairy Tail teams can truly fight each other by giving their all. In fact they might fight each other even more gleefully than strangers. The only obvious problem here would be Gray and Juvia. Elfman and Mirajane as well, but on the other hand Elfman is not the real member of the A team.
It shouldn't make sense that they can substitute members in and out of a team like they suggested..
But then again, this committee doesn't seem to give a shit about any of that.
If it's not Isuzu-chan Mii~
The committee is obviously just going to steal everyone's power anyway, so they don't care who competes.
If we're going with the "steal magic" theory, then FT-A team could be among the few with enough power left to fight the bad guys thanks to their separate (hidden?) compartment (2nd Origin).
If it's not Isuzu-chan Mii~
Or maybe not steal. Maybe they'll just analyze all the contestants and then make golems or something that duplicate their powers.
You know, the usual shounen reasons the badguys trick the good guys into all gathering in one place.