Unless the manga changes the story, Asley realizes it's Tzar right away.Surely he'll recognize the two headed snake with mana signature or something right.... not the stupid mistake ally for enemy trope again right....?
You're right. It's a detail that's been kinda glossed over in the manga, but the monster attacks have been becoming more frequent as of that point in the story. Things were kind of slow going after Asley left, but then a handful of years later, the monsters started returning in larger numbers than ever.So Reed is still alive after he sent Mana through the gate. Rina would be pleased to hear her siblings are still alive and kicking.
I presume this big battle is something that hasn't happened in a long while or else they couldn't have survived this long without further help. And a big battle is happening now because the monsters are probably being directed by someone or something to attack, this is no ordinary everyday encounter with monsters who happen to be roaming in the area. Gotta find out who's behind it.