Is there a way to setup livesplit to create a video file or something like that for a run that has already been completed to be edited into the video of the run? Essentially allowing me to frame perfectly make my splits on a video file.

you could separately record your livesplit window with another obs running in the back while you record your game on another obs but even if you record it separately so you can match the start and/or end its not like you are splitting every split frame perfectly anyway so it still wont line up.