I am very sorry for your loss. My snakes are members of my family, and I'm sure you felt the same way about your little one.
High rates of mortality in snake hatchlings is a fact of nature. Without a necropsy, it will be very difficult to determine the cause. Knowledge can help prevent future problems. Thus, I agree with the other recommendations for this if you can arrange and afford it.
The timing of the feeding and the remainder of the lump in the digestive tract would suggest a higher probability that something about the food or the feeding was responsible, but it doesn't prove it. If some toxic substance had gotten on the pinky without your knowledge, then the cause would be more certain.
I believe there is a table of recommended times to thaw different sizes of frozen mice somewhere on the Internet or in one of my books. I can't locate it right now. If someone can provide a link or post this information and its source here, we'd all be grateful.
I remember thawing frozen pinkies for 15 minutes inside a plastic bag immersed in warm water. You mentioned thawing for 5 minutes, and didn't say whether it was at room temperature or with warm water. Should you get another snake in the future--and we sincerely hope you consider that at some point--you might try thawing pinkies longer, as I do. I don't know that a partially frozen mass inside an outwardly thawed pinky can cause death, but the results can't be good.
In any event, do not be too hard on yourself. This happens with hatchlings, and more often than not it is not your fault if you are a caring owner, which you obviously are.
With sympathy,
Doctor Mike