thats true but no worries I´ll try my best.
1) mopidy has nothing like a dlna server. I am using https://github.com/binhex/arch-minidlna. But I switched using Plex now.
mopidy is only for music. I am using it like a radio station for multiroom audio controller.
You can build chromecast audio groups or if you are not able to do this you can use mopidy and snapcast for synchonus audio streaming.
with mopidy you can access spotify, youtube, soundcloud and local media at once, you can search over all this services and play media over all connected divices.
2) dlna lacks if you use transcoding. switch that of use mini dlna. mayme there is an extension of mopidy to support dlna. but easyer is to use the stream.
3)on chromecast you can stream http stream to the audio group and youll have synced audio. If you dont you can use snapcast and snapcast devices.