Thanks for the replies. We have an end of the story, at least for now.
After all the emotion settled (and the dog Dd liked was on it's way to a new home probably as I was writing), I figured out that our problem was really animal deficiency of a general nature, which I could remedy by letting the cats have more access. We allowed them into areas of the house where Dd tends to be, as well as made her bedroom cat friendly for the nights. Yes, we've had a few litterbox accidents which I take to be protests when Dd's not keeping up with the box in her room. They have earned nearly free run of the house most of the time and they are lovely to have in our midst.
Best of all, they follow Dd around, always happy to have a scratch between the ears. One in particular never lets her out of his sight, so her yearning for the companionship of a dog had been satisfied by the cats.
I still have to deal with the fish tank, but I wanted to report back on our progress. Thanks again for all the advice.