I have in a website a directory of musicians, music groups and institutions (luthiers, concert halls, etc...). According with the official documentation of schema.org about MusicGroup, I could include also a solo musician.
My questions are:
Should I assign "Person" to solo musicians and "MusicGroup" to music groups?
If I assign "MusicGroup" to solo musicians, his/her thumbnail will be displayed in the Google search list as rich snippet? (I guess if I assign "Person" to solo musicians it will be displayed).
The same as question 2. but with music groups.
The same as question 2. but assigning "Organization" to institutions.
I am very interested in show the thumbnail in the search list, but also to give a logical and correct semantic syntaxis.
To expand on my answers to your questions 2,3 and 4:
Google only display thumbnails for search results in a few rare cases. My suspicion (Google's decision-making process is not public) is that thumbnails in search results are more dependent on having a very high-ranking website and therefore - in Google's eyes - a highly trustworthy and reliable source of information, rather than on the particular schema.org "type" you use.