author here. thanks that it's helpful.

> does anyone know if it's technically possible to cast a video to Chromecast, but leave the audio playing on my local computer? (Necessarily involving some kind of audio delay.)

yes, it's technically definitely possible. one way is to use ffmpeg to remux the video alone and pass it on to the chromecast while playing the audio along locally. you can use the chromecast api to fetch the playback location of the video and use it to sync the audio locally.