Hey Vertus,
1) --cpu-shares is actually a soft limit, only enforced when your cpu is in use. If its idle for the most part, the container can use 100% no prob. I should update the description in that section.
2) beets alone (the one in the CA) is mostly for music management and tagging, I think you're thinking of this: https://github.com/Neurrone/beets-audible in which case the best way to use it in unraid is the docker-compose setup descrbed in the repo. You can manage your own docker compose files (and have them autorun/etc) with the Compose Manager plugin.
I just use audiobookshelf for metadata management though, works well enough for me.