How can I monitor a specific folder and know what processes access its contents? I'm working with a complex Chrome Extension. As I debug it and make changes, some other files are created and modified.

I know for certain that are not modified by me and the extension is not being run.
Maybe it's a spun off process from the IDE... I don't know.

I've tried my luck with lsof without great success. I'd like to constantly monitor that directory and after a while check up on it and read something like "X file has been created by PID Y".
Take a look at fs_usage - you may be able to parse the results it gives you to determine the PID that created the file of interest

sudo fs_usage | grep [file]