I will try to answer best of my knowledge.
1) If you have only one cache server its not mutli threaded so it might go to queue / or may go and fetch against database or an extract is my assumption.
2) In a multi threaded environment each process talks with all cache server processes. Coming to Cache master, There is no primary or master ,Here cache data is not in sync but all the process have the ability to check the cache on all other cluster process before going live / or against the extract.
Here is an example for Tableau Presentation.
3. Yes there is Per process Vizql Cache , But in 9.0 as I said above it's shared so that all the process even from different cluster can talk to it and share the cache.
Experts please correct me if my understanding is wrong.
Tableau Server Processes