My dear Linda, I knew you would share this story sooner or later. Still, I wasn't prepared. I had to wait for my tears to stop before I could start typing this.
Please, please know that you did all you could to help Shadow. I am appalled that you couldn't get emergency veterinarian care for her, but I have doubts it would have saved her.
I've shared my life with dozens of cats over the years. Abandoned kittens often have serious health issues that you can't see easily. Animals, especially feral ones, are adept at hiding weaknesses. Her sneezing and other small symptoms would not be alarming to most.
She was sick when you found her. She would have died a much worse death without you. You gave her joy during her very short life. That is important. Focus on that. You gave her joy. You loved her. She loved you.
I'm not a vet, but to me, it sounds like she may have had pneumonia that led to kidney failure. That's a lot for a tiny kitten. When I was a child, our cat got pregnant. She had one kitten - one very large kitten. One of the most loving and joyful kittens I've ever known. The only symptom that something was wrong was a runny nose. She lived a month and died in my hands. We had the money for a vet. We had a good vet. We didn't know her runny nose was a sign of congestive heart failure. I was nine years old. I cried for days. But I was so grateful for having known that precious being.
You and Shadow had a limited time together, but it was a very special time. Yes, you are devastated, but you are also extremely lucky to have known that precious baby, and she was very lucky to have been saved by you. Don't ever think that you didn't save her. You did. It was her time to go, but she lived longer than she would have without you.
I firmly believe that pets come back to us. I've had it happen many times. A beloved pet passes over, and a few months or a few years later, I see that pet in the eyes of another animal, and I know he/she has returned to me.
Look for Shadow. She will come back. Some day she will return to you.
I am holding you close to my heart, Linda. And I am so grateful you shared your story with us. Our stories are what make us human - even the tragic ones.