Their first plan can't work because it's too simple for a climax. After death and destruction, you can't defeat the villain like turning a key. So I was plugging along putting in complications: the villain noticed the loophole they were using, and plugged it, meaning his defenses work better than expected, and our hero has to find a second loophole. And one of our heroines have to put everything on hold for a time, because it turns on time and ages, and --
At some point, I looked back at the original plot and went, O.
You see, it entirely turns on using the magical virtues of a jewel, replacing the one the villain intended to use with the one the heroes brought, thus breaking the mechanism. And trapping them in the middle of the villain's lair with no escape. A villain who has already shown himself willing to murderously attack them despite their youth.
No. The other heroic figures would NOT have let the real and present danger that the young characters are in cause them to bend that far. A dangerous mission, they could accept. A suicide mission?
So, even if the plan has crash and burn, it has to be such that if it worked, they could have escaped alive.
I suppose it's time to brainstorm. Escape? Depower the villain? Imprison the villain?