Act 2 plays out the same until the very moment Yuri is about to stab herself, where you save her in the moment and then she implies awareness of the 4th wall and the screen immediately fades black to 'Monika' thanking you again.
Monika thanks you in an OK-box for saving Sayori and the game moves to Act 2. The way you save her isn't by anything particularly key that MC says but by Monika having a change of heart and ceasing to amplify her depression. You restart the game and Act 1 plays out exactly the same up until the last conversation with Sayori. In the beginning, 'Monika' tells you that you're going to go back and save everyone. I'm not sure if there's a possible headcanon that makes it work: It copy-pastes almost all of Acts 1 and 2, it doesn't handle the resolutions to the characters' problems particularly well, and the plot is an incomprehensible mess.
The reason for my negative verdict here is that it's kind of a strictly worse Doki Doki: Salvation. I feel bad saying I don't recommend this mod, because I puke at the thought of some of the mods that I gave ambiguous verdicts to being better than this one. Our Final Heartbeat is a Doki Doki Literature Club mod with the premise of going back through DDLC and averting everyone's deaths.
