About that smirk...(spoilers)

[Cilia]

snip.

The problem with this line of thinking is that it runs on confirmation bias, a certain psychological fallacy where you interpret things so they line up with your hypothesis.

Tataru smiles and says "Well, I guess this is goodbye. ... wait, that feels too final. "See you later," I guess?" and goes on her way. That's all it meant. The trope wasn't suggesting betrayal, it was suggesting Tataru was a true friend you could trust and who was sad to part ways with you, and who chose her words poorly and corrected herself. You're twisting that to line it up with your "Tataru is a scheming, backstabbing rat" theory, which there is no concrete evidence for. Without evidence, your theory has no ground and can thus be disregarded.

As for how she got the materials for Y'shtola's new outfit... I don't think it's that big of a deal. She works odd jobs, and is stated to be pretty good at them, so having some spare change (plus whatever was left in the Scions' bank account after 2.5.5) makes it plausible she'd have the funds. There are chimeras outsize Azys Lla, they're just extremely rare and dangerous to fight (see: Cutter's Cry, A Relic Reborn, Go Go Gorgimera), so having the hide is possible (if a little improbable). I forget what her staff was made out of, if it was even explained beyond "it's rare and expensive."

You're just throwing out a wild hypothesis with no real evidence to back it up, which makes it very easy to shoot down. Honestly, I think all these "Tataru is evil" threads are coming up because we don't have much to speculate on after 3.0 except the Warrior of Darkness and people can't accept that there's a cheerful character when 3.0 is all doom and gloom. (Hint: we need Tataru to lighten the mood, lest we drown in our sorrow and despair.)

TL;DR ver.
What you're doing is this:

... which is an extremely, extremely poor argument.