I think this can be done manually, pretty quickly, without breaking anything.
You’ll need to do three things:
1. Select all keyframes and use the “Time-Reverse Keyframes” keyframe assistant.
2. Reflect all layers in time. If a layer BEFORE reversal starts 10f into the comp from the beginning, it now needs to end 10f out from the end. If your layers are all trimmed to their first/last keyframes, you can just move the layers in the timeline. If not, you’ll need to move the keyframes instead, following the same logic.
3. Any footage layers need to be individually time-reversed.
To more easily accomplish step 2, throw a text layer in your comp and add this expression to its source text:
Set your comp’s time display mode to frames. Now, when you put the current time indicator on a spot on the timeline, the text layer will tell you what its corresponding “reflected time” should be. So, for example, in a 900-frame comp, put the CTI at the beginning of a layer that STARTS on frame 100 (or has its first keyframe there), and the reference text will tell you that layer should END on frame 800 (or have its last keyframe there).