I took a look in on the kittens this morning, and the grey tabby had what I thought might be a loop of intestine herniated from the umbilical opening. I called the vet, who asked me to bring all the kittens and Dolce in for an exam. Sure enough, the one kitten had a birth defect, and the vet recommended that we not let her suffer, as there was no way to correct the problem. So I named her Pasha and held her as she passed away.
The other four kittens were examined, and look to be in good health. No cleft palate or any other obvious problems. The one kitten with the funny back paws has all the necessary joints and bones (according to the vet) so the deformity may be something s/he grows out of. Nonetheless, it doesn't seem to bother or impede the kitten at all.
The surviving four still don't have names. I will update as soon as we name them.