I think we're all familiar with foreshadowing; but we have to remember that it can also work more than one way. As I mentioned earlier, I personally wouldn't have thought much of Tataru's smile had it happened at any other time. All the quests leading up to the Scion's downfall put special emphasis on small matters that would make the impact that much harder, such as exploring Yda's character a little bit more since you wouldn't be seeing her again. As others have mentioned, what she did was a rather typical anime trope suggesting betrayal. It's not really a matter of reading too much into it; the first time I watched the scene, that's what I automatically thought it was. I'd nearly forgotten what a Tataru was before then, honestly.
And given some of the revelations that we are left to piece together from the final area in the game concerning some pretty major plot elements, I wouldn't say that theorizing on certain aspects of the lore or characters -- which is all we're doing here -- is uncalled for. The devs left subtle hints in for major plot elements far earlier in ARR than I'd thought because I didn't start in Ul'dah.
Kaiser-Ace: I wasn't suggesting that she didn't have help. Quite frankly, I think she did have help. I just wonder who helped her, since the Warrior of Light is called upon whenever someone has a mild case of the sniffles.
As for what motive Tataru could have, that could be anyone guess, if the theory turns out to be correct. We weren't really sure what Lolorito's involvement in all this was until well through Heavensward.