Well, I don't exactly recall if the MC and the gang had been in GDC enough time for Cosmia & Comet to comit that type of fraud (and having no reasons to think they did it, except there was someone behind them) so I doubt they would be the culprits. Crescent is a good candidate, but she seems kind of fixated into the MC, so why would she do that to all the gang?
Last but not least, Amanda wouldn't have looked so suspicious had she not confronted Melia before entering the party Bladestar was going to bomb. And if I remember right, she knew Melia was alive and well, as she had stolen her invitation to the party. So she might have thought of making the gang all in the Tournament just to take them out the party and lucked out with Melia losing. Erin clearly wasn't on her VIP list or maybe she has an important duty to perform inside. But of all of them, I think Amanda would be my first candidate, even when not stopping to consider some Stormchasers agent, the Puppetmaster (he did send the invitations to Erin and Melia after all) and so, an so.