Seems like they're preparing a new part of Time Temple.

Who screwed up the summoning?
It wasn't a member of the Black Wings.

...then who was it?

Herpderp. Nexon's incompetent. That's hilarious.

just making a point that Nexon has a thing with not finishing storylines, or making them end poorly.

He tries to dissuade Cygnus from helping Mercedes? That's news to me. I'd love to see a source.
What I do know is that he:
a) Tried to convince Cygnus not to attack LHC. She didn't listen. The Knights got blamed for Ifnia's death, and Von Leon ended up joining the Black Mage.
b) Tried to convince Cygnus not to go after the World Tree. She didn't listen. It turned out to be a trap, and she ended up turning psychotically evil.

Along the way, they are attacked by the Black Mage’s henchmen. In town, Helena tells Mercedes to go to Ereb and ask for cooperation from the Cygnus Knights to fight the Black Wings. Nineheart is reluctant but Cygnus wins him over.