Mac refusing to shut down because of finder?
There are 2 tools I recommend to use to narrow it down.

One is called EtreCheck (you have to download it)

The other is built-in system check from Terminal. The following command runs system diagnostics and creates a text file. 

sudo sysdiagnose -f ~/Desktop/


It might take 5 minutes but creates complete report. 

We could also just look for it in the Console.
For that you need to note exact time when you try to shut down.
Now go to the time stamp in the Console and see what it says.