Can you see other users' session variables in MySQL?
As of MySQL 5.7 the performance_schema can provide this information.


  Is there a way to see the SESSION variables of other active, connected users, from a superuser connection ?


You can get these from performance_schema.variables_by_thread.

You will need to know the Thread ID. The ID from SHOW PROCESSLIST is Process ID. Knowing the Process ID you can then look up the Thread ID from performance_schema.threads.


  is there a query or view that would show me which active sessions have variables set differently from the global variables?


You sure can.

SELECT VARIABLE_NAME, gv.VARIABLE_VALUE, tv.VARIABLE_VALUE
FROM performance_schema.global_variables gv
  JOIN performance_schema.variables_by_thread tv USING (VARIABLE_NAME)
WHERE gv.VARIABLE_VALUE &lt;&gt; tv.VARIABLE_VALUE
  AND THREAD_ID = ?;