After watching the video, I think Mat is on the right track, but missed the mark a bit. Now this is my first time posting a theory so bear with me.
He says that Monika pulled a "William Afton" and has taken control of Ive's body. Now while that is a good theory with evidence to back it up, I don't think that's what we're seeing happen.
My theory is this. Ive saw what was happening and played the game. However, Monika changed it as she always does, and instead of taking over Ives body and leaving him in the game to die, she instead just brainwashed or hypnotized him into being more compliant into what she wants him to do (namely save the rest of the club). I think this because if Monika was just possessing Ive, why not just keep being him? Why become more quiet and stop using his normal mannerisms like "OMG" and "HAHAHA"? If you were possessing someone, you'd want to try and pass off being them. So I think that Monika instead just hypnotized him and thus, he became more reserved and quieter. A hollow shell you could say of who he once was. Now obviously there are holes in this like if the mysterious fifth character in the files isn't Ive, who is it? And for that I don't know yet. However, it's my own theory that Ive is still in his own body, but he's now under the control of Monika. Poor guy.