Only a character like Tara can make you fear park benches and windows (I know someone who did fall out of a window and needed many stiches, I taunted them about thin sheets of glass for years but now I know to stay away or stand close, depends on how I feel.) So lets shove someone else on the bench.
Everything happens the same, the argument and storming out but instead Tara went to the magic box and Willow went to the cultural fair. The monks messed up and told Glory that Willow was Tara because names are hard to remember for minions if they aren't attached to the splendiferous one. Tara does run to the fair and tries to do a spell to stop it but is too late.
Do you think Tara would take the same course of action as Willow and turn to the Dark magics, be it for revenge or trying to find a cure?
I'd say no. I think Tara would go to Buffy or have a Buffy weight of the world comatose situation out of grief and self destructive hate at her failure to save Willow. Be awesome to see Tara throw around some powerful spells and be the scoobies only witch for a time being but I don't think dark magic would ever be an option for Tara. Capable of stronger stuff absolutely however her moral compass is strong, ignoring being part of the resurrection.