...so the shelter called us back yesterday evening. We explained the situation and our concerns. Shelter worker told us that it often takes 3 weeks before the rescue dog starts getting comfortable, and they recommend 3 months before we give up. But our minds were pretty much made up, so we're currently scheduled for returning Freya on Sunday afternoon.
Didn't feel right about it. Little Miss Field Day thinks we're being premature, as well.
But... we slept on it. And The Boss is having second thoughts. Freya is also much friendlier today... she's been in and out about a half dozen times today and tonight, and starting to interact with us (playing catch, playing tug of war, curling up next to one of us, and so on). She just came in as I'm typing, and came right in and sat down next to me.
It looks like we're going to give it a little more time.