Thanks for everyone's input and support. Piper was breathing better this morning, so I took her in and the vet did a thorough exam. We decided it was safe to go ahead with the spay, baby teeth removal, microchip, and I insisted that IV fluids be administered (I think it's helped her come out of the anesthesia more easily).
She's been home about an hour and has eaten well--in two small sittings. And she's not protesting her restrictions at this point, so I think the pain meds are keeping her comfortable and relaxed. I'm so relieved that things have gone smoothly for her and now it's all done! No accidental pregnancies, etc. Whew!
She's pictured here enjoying a second small snack before heading back to bed.