The “Unable to find mixer control Master” is a known issue with the stretch RPI rasbian, as it does not have a Master but a PCM. I believe this brach fixes that part, but needs testing;
https://github.com/MycroftAI/skill-volume/tree/bugfix/picroft-mixer
However that is not the issue as it only prevents you from setting the volume.
The issue is I believe that the messagebus falls on it’s knees and therefor the other services including the skills service are halted before all downloads are ready.
I see some ALSA invalid sample rate stuff, so perhaps you should have a look at this thread;