Yeah- I was just so angry at the vet this morning. She finally called me back and was very laisse faire about the whole thing, which stung a bit more. I did consider necropsy- but as other's have pointed out it could be so many things that even a necropsy may not tell, and I am on a really tight budget right now, so I opted not too, mostly just because I cant right now.
She was probably very 'laisse faire' about the whole thing because this is not a rare occurance, in regards to the death of newborn kittens. As for 'finally' calling you back, vets are responsible for surgeries and calling back the in-clinic patience. Working at a vet clinic I know how busy it gets and how hard making call-backs can be. A lady with some dead kittens frankly is on the bottom of their priority list. Like you say, the mother cat was in, from what you can see in plain eyesight, good condition and thus there was no reason for you, or the vet, to think that intervention was required.
Anyhow, the most important part is realizing none of this is your fault, nor the vets, and it is just a part of nature. Have you considered nuturing the remaining kitten yourself, being that there suggestively was an apparant problem with the mother?