I have hundreds of .kdenlive projects that were created with version 17.12.3. However that version has bugs with rendering that result in poorly synchronized audio and video. The new version seems to work better, but I don't want to manually re-render hundreds of files, I want to write a script. So my question: can the current version of kdenlive_render handle .kdenlive inputs from 4 versions ago?

Hello,
For rendering kdenlive is actually not involved, it's melt doing the job. Melt has improved but is fully backward compatible,
so you can simply call "melt your_project.kdenlive -consumer avformat:your_project.mp4 crf=20" (I let you pick the output format and encoding parameters to your liking).
Good luck!