is it possible to cast from iPhone to apple TV and HomePod? I want to to be able to watch my Apple TV movie on my TV. I currently have Apple TV connected by HDMI to my Samsung TV and I also have a HomePod. I want to be able to stream the picture to the TV (possibly using the Apple TV device) and stream the audio to the HomePod. Is this possible?
Yes. 

iPhone will airplay to the AppleTV which can select the HomePod(s) as the audio from AppleTV.