A couple questions… do they want a finished video or just clips to edit into their promo? Finish editing is a real time suck, sourcing/licensing music, speed ramping, time lapse processing, special effects, exporting, uploading, etc.
I don’t think flying over a crowd is a good idea under any circumstances. I always assume a possibility, however unlikely that it could come down.
I‘ve witnessed drone companies doing festivals I’ve attended and they tend to avoid flying over a crowd at all. It’s possible to capture footage by flying the side lines, corners, behind the crowd, behind the stage. The concert promoter would feel more comfortable if you were to avoid the crowd flyover, in which they would share liability regardless. Of course, there may be a few people under your line, but not a crowd. You could cordon off an area on the perimeter temporarily with the help of security staff, from which you could fly straight up to capture a scene. There’s always a chance something could occur. I’ve crashed enough times to know it.