What's a couple decades with a dying witcher school, the world's most powerful sorceresses and the ability to teleport to the dimension where magic was birthed?

I'm pretty sure Yenn or Triss mention how trivial it would be for them to figure out the potions and transmutations required to make the trials happen but the Witchers are very, very against an outsider touching it all. The original plans were made by a sorceress in any case.

I'm sure they'll have a perfectly reasonable way of explaining how she went through the trials.