Other possibilities are the Infinity Ball, which Scarlet is implied to have knowledge of. If this is the way they go then it’s possible that Scarlet isn’t from this timeline, and comes from one where Mord has already attacked and the Pale Tree has been taken over/destroyed.
This would explain her ease of control over the Steam Creatures which are seeping over from the alternate reality created by the Infinity Ball, and would be a cheap but plausible explanation for why Scarlet popped out of no where.
When you state that “Scarlet popped out of nowhere”, if you mean the actual physical body then you are completely wrong because we know exactly where that body came from: The Pale Tree
She was born just like any other Sylvari and her name then was Ceara
Ceara emerged from the Dream of Dreams with an innate fascination with the interlocking systems of nature, and a determination to forge her own path independent from the Mother Tree. When the rules of the world prescribed her one destiny, rather than struggle against them she would simply change the rules.
She also spent 11+ years learning what she knows.
However if you mean the persona of “Scarlet Briar” then there is a possibility that while in the isolation chamber her mind didn’t just snap with ill gotten knowledge (it is also from the experiment with the chamber she learned of Caithe’s secret imho since it is stated she learned the grand design behind the Slyvari) but was taken over by another force. However everything stated so far points to that not being the case:
Ceara entered the machine, which was an isolation chamber that removed all physical stimuli to let the mind run free, and during the experiment she saw visions which she believed to be the Eternal Alchemy.1 She saw Tyria and the Mists, she saw the insurmountable challenge of the future and her people’s part in it—the sylvari of the tree and the Nightmare Court, Caithe and Faolain—all part of a grand design. And for the first time in years, she heard the voice of the Pale Tree, begging her to go no further. Ceara pressed on regardless, which broke her sanity while allowing her to think in different ways that allows her to progress in ways traditional thinkers can’t. It is unclear whether she actually saw the Eternal Alchemy, or if it was merely halucinations.1 Upon emerging she killed Omadd and took her new name, ready to burn the world.
http://wiki.guildwars2.com/wiki/Scarlet_Briar
According to that (which is based on the short story of her life and what she saw in the chamber) the knowledge to do what she is doing has always been there, it was just something no sane person would ever think of doing with that knowledge.