My theory here is that the game world demands that a villainess of some kind exists, so since Elena has shirked that duty, a new villainess has appeared, intent on taking out the old one since she's no longer fulfilling that role.
Of course, this raises the question of "who else would the villainess oppose, but the protagonist?" Some inconsistencies are starting to emerge between the world's current state and the state that the game "should" be in (Lance will seemingly not have a falling-out with Elena as destined, the water spirits attacking, et cetera). So, I think it is possible that the game world may be starting to treat Elena as the protagonist, rather than Tiara. Indeed, perhaps the game may even put some amount of pressure onto the idea of actually having Elena and Lance get married -- something Elena will have to overcome. In other words, Elena may have to put thought into the idea of "escaping the Lance Route she put herself onto," so that she can finally get what she's wanted this whole time instead?
Let me phrase it as such. There may not actually be a Yuri Route for "Tiara's story." However... what if, instead, the game world started telling "Elena's Story" instead of "Tiara's Story"? Perhaps "Elena's Story" DOES have a Yuri Route, which just so happens to have Tiara be the love interest.