At this point MC is literally just a background decoration. He should've asked her a long time ago who/what she is and why she is an impostor (and then report her to the police if deemed necessary). But nooooo, he's scared. He's shy. He's a damn vegetable.
But he did ask. She's been avoiding the question.
Also for crying out loud there's nothing achieved by not showing readers whether she actually started the fire or not. It's a nitpick but so many series do this thing where we follow a character the entire time and there's no hint of them doing something sneaky (like planting something that could start a fire). Then all of a sudden the thing does happen and in a flashback it's revealed they did it all along.
Oh my FUCKING god. YES! That's just a specific case of authors using flashbacks to fill in blanks in information rather than actually telling the story right the first time. I think I've seen a flashback done right exactly ONCE (I think it was the last season of Shinigami maid, right near the end), where they said something off-screen (You know the kind of scene: Where they're clearly sharing some big revelation but we can't hear it), then avoided telling the audience right away to preserve the tone for a happy moment that was about to come. After said scene was over, we flash back to the message to finally let you know what was up and move the story along.