As stated by PP, this is no different than any other major event or function where a mass of people are exiting at a given time (example: convention centers, theaters, arenas, stadiums, etc..). In addition, any safety precautions, fire code requirements, and emergency exit plans, etc. would have already been put in place as required by building and public safety codes before the amphitheater even opened. Not to mention, the amphitheater opened in 1998 so I am sure it has been through MANY fire code inspections over the years to ensure the right safety and emergency exit procedures are in place.
If it makes you feel any better, as far as emergency exits at this venue, the amphitheater DOES in fact have additional emergency exits that could be opened in the event of an actual emergency evacuation, one of these exits is located on the far left corner of the amphitheater that leads to a backstage exit area. So the exits in the back and right side that are used as normal exits during a regular show are NOT the only exits.
As stated by PP, if you feel that there was a legitimate safety concern at the time that you were there then you should have reported it at that moment to a manager in the area of the show or Guest Relations. If you insist you could send an email to the WDW Guest Communications email listed by PP above but this email is more so for general complaints and issues, it will be nearly impossible for them to validate your concerns from that particular night and you will likely a receive a generic corporate response if any.