No plot hole.
In Season Five, Willow learned how to speak telepathically, we saw that in "The Gift" (but only Spike heard her.)
In "Bargaining", Willow was guiding the gang telepathically, apparently now speaking to several at once.
In "Two To Go", Dark Willow tells Dawn (while they're at Rack's place) "Shh. You don't have to talk. Just think real loud; I can hear you."
In "Grave", she speaks to Anya while held in Giles' stasis field, then speaks to Buffy again later while they're some distance apart.
It seems fairly obvious that, like everything else she attempted, Willow worked at improving the ability until she took it well beyond that first basic communication in "The Gift". Unfortunately, she seems to be permanently on "receive" now, which might be one reason she looked so tired through the first part of Season Seven - having to tune out all of those thoughts so that she doesn't go mad from them (the same way Buffy was being driven mad in "Earshot") would have to wear on her, but it's something that the "real" Willow would do after her crash, since she's basically a decent person. Buffy does exactly what Dark Willow told Dawn to do: she "thinks real loud", and Willow does, indeed, hear her. For the three-way conversation in the kitchen, it's Willow acting as a "substation" to allow Buffy and Xander to speak through her.