OK, well keep in mind you are jumping straight to an advanced setup here.
There is Mopidy-YouTube. I don’t use it. I don’t know the current state but there’s an active maintainer who responds to Github issues and also on here.
This is not possible. Google Cast is closed-source and I’m not aware of any usable library for it. Thanks Google!
Presumably you’ve seen How to set up a full house audio system with X streams and Y speakers ?
You should get this working in stages:
- Install Mopidy and Mopidy-MPD (from apt)
- Install Mopidy-youtube (from pip for the latest version, I think…)
- Install Iris webclient for Mopidy (from pip)
- Get local playback working
- Install and configure snapcast. They have great documentation.
- Get remote snapcast playback working
- Repeat for each system
- Install and configure HA for the multi-room aspect. I have no experience using HA but others on here do.
I don’t think you need to install mpd for what you want.