I have serious problems setting up Master Audio in my scene

Play on awake? That's not a feature that exists, if you're talking about checking that checkbox on the Audio Source itself. I disabled it as it makes no sense with this product. If you want a sound to play as soon as the Scene starts, use Event Sounds script on a Game Object in the Scene, with Enabled event. Have you gone through the Quick Starts on the documentation website?

You will need to ask specific questions and I can surely answer them. You haven't shown any screen shots of what you're talking about either.

The final volume of a Variation played is: Variation volume, times Sound Group volume, times bus volume, times Master Mixer Volume. That's why there are 4 volume controls.

Clips only loop if you click the "Loop" checkbox.

Thanks,
-Brian

Music takes more resources if its streaming, otherwise no difference in resource usage. 99.9% of the code is event-driven in Master Audio so that it's efficient and lightweight as possible. If you have no need for Playlist features then you could use a Sound Group, but Playlists are meant for music.