After all that worry, they remembered to try Dispel Magic.
They only had one spell slot left, and if they did a long rest, the player in the box would die from the d4/hour ability score drain. I decided to go with 1/hr to give them a chance. They had bardic inspiration to enhance the Dispel Magic, which had a DC of 19.
...and they made it! Whew!
They then took on the Pitchblade Barbarians and it was trivial due to some failed saving throws on my part. One barbarian was stunned by the monk for two rounds and they pounded on him. The other was turned into a fish by Polymorph. Meanwhile the druid and bard summons mephits and animated objects to deal damage. Wasn't fair at all.
...so that night the barbarians snuck in to get revenge! Surely this time they would make their saving throws?
Turns out, not so much. But the bard just wanted to sleep, so he cast Calm Emotions and the Druid cast Suggestion, and they got the angry barbarians apologizing for interrupting them and they'll come back to fight them after the games in public where their redemption for so easily being defeated can be more visible.
I was getting excited about my hasted, raging, frenzied barbarians with 4 attacks a round, but action economy took them out. We'll see how the semi-finals go!