[SPLIT LIP]

Your theory doesn't quite match up. The creature's blast was obviously the first strike, Kouta and Mai were not engaging it before that. And if blonde-Mai was trying to stop him from killing the monster, why did she remove the lock while it was open? We've seen you have to put it back on then close it to revert back. If she wanted to stop him from fighting, she would have closed it. The only purpose the lock serves removed and open is as an attack, and she pointed straight at his sword. Removing the open lock does nothing but start the finisher. And of course, there's just the general ridiculousness of why be so cryptic if she's tryign to get him to stop. Kouta had no idea what was going, he had no chance to. If anyone's to blame for Yuuya's death, it's blonde Mai, for being a generic cryptic sage.

Also, "MUST SAVE THE GIRL LOLZ" is kind of radical when he's, y'know, fighting for his life. It's a really harsh and irrational thing to put on a character who's just trying to survive and protect his friend.You're making Kouta sound like an idiot when, from his perspective, he did the rational thing to do. He didn't even know the attack would kill it.

I agree the Invess was Yuuya, he was wearing a yellow shirt afterall, and the way it runs kind of looks like it's chsing after them to get their attention, not to attack, but once in the real world it turned hostile, possibly frustrated or losing control. Also, it's voice is not typical monster voice, but actually sounds more human than mindless monsters are usually given.