The way it works is that you put components together, and when either the timer reaches zero or your health reaches zero, the spell you built becomes your new spell. There isn’t a failsafe wave, you just built a really effective spell! I should probably work a bit on conveying that idea more effectively.
As for Plan B… I imagine she needed some sort of artefact for Plan A, and while she doesn’t strictly need it, saving the day becomes a lot more dangerous without it.