Until there is substantial additional areas (garages/multi-level) provided, the various options (remote parking with shuttle service, car pooling, bicycling, etc.), each contributing small bits of relief, seem to be the only immediate solution. One I have not heard about is perhaps promoting off-shore attendance (private or party boats). Again, only a small, but additional, contribution to the solution.
As for the merchants, in addition to the theatre offering special concert night prices, the local restaurants might offer special “take-out” or “brown bag” dinners and/or special “after the concert” dinners/snacks and beverages. Also, sidewalk kiosk might bring in a few dollars.