You can definitely buy tickets online and it's probably not a bad idea since the theaters are not large and they are limiting the number of people they admit. Parking is mainly street, so I'd either park at Loyola and walk to the theater or plan on spending some time finding street parking.
Yes, I would buy tickets in advance. In case the theater has limited seating capacity, that would gurantee your time and space. Also doing Uber will eliminate the need to hunt for parking.
All seats are general admission.
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