1. Just don't accept any bill over €50 (this is common in shops too, no one expects to be paid for groceries or a beer at a festival in €100 bills). Most people are still pretty skilled in handling cash, although this is a declining skill. We've been doing cash transactions for centuries, we're still pretty good at it. The occasional slow-down is more than made up for by not having to charge or top-up your bracelet or what have you (you come prepared to the festival with cash).

2. A negligible amount for a vendor on a festival.

3. Not a problem on a festival. Booths tend to be manned by two persons because of other safety factors anyway, so staff stealing isn't an issue, and any organised criminality on a festival is usually limited to (relatively) low risk, high reward crime: pick-pocketing (smartphones in particular).