(Get out of here, Jax, this thread is totally not discussing your secret identity /jk)
There are other theories that he's Icathian (by Majestic Raven), or Ionian (by a wrenchman!). The Jax threads are countless
I'm inclined to believe that he's Ashram though. Jax wears purple robes and does/did magic damage. What other group of magic users wears purple, face-concealing robes (not PCM)? Also, Jax's logo wouldn't be a bad representation of the League. It's a number of city-states, represented by dots, contained within the League, represented by a bigger circle. Afterall, it would be within the League's interest to have powerful champions secretly fighting in its favor.
As for Jarvan - He disappeared in Kalamanda, north of Mogron Pass. Several issues later, there was a top secret Zaunite shipment to Noxus, and then there was a massive fight in Noxus's merchant district. If the interpretations of the Kalamandan note are to be believed, "quarter" refers to Jarvan IV, and "quarter to clutter" could mean he was in the fight in Noxus. After that ... he was either recaptured, or he actually escaped. His reappearance in the last issue could actually be him.
If he were fighting the Void, though, he'd want Kassadin on his side. There's no indication of that happening ...