Still, it doesn't make any sense for her to fall for Tadokoro san at this point
Why not, Tadokoro is adorable and has been super nice to her, even when she hasn't deserved it. Now, I don't think she's actually gonna try to get with her, because A: Kirigaya is not really self-aware that she's into girls (yet), and B: even she knows better than to summon the wrath of Nikaidou again.
And as you say, she already has a girl, even if she doesn't realise it yet.