So sorry for your loss! Especially after all the work you did to protect that egg from her inexperienced mother. Thank you for sharing your experience so we can all learn from itWell, I just got through a life lesson.
My eight-ish month old silkie went broody. I had read that young broodies can have a bad go of it, but it worked out with my other silkie momma, so I let her nest. From the get go, there were problems. She would get up off the nest often. Only one of the eggs fully developed and it took 26 days to hatch. The day of hatching, she kept getting off of the egg. It took 30 hours for the chick to hatch from when it started. I had to keep moving the egg under my mature broody to get it warmed up. Finally, the egg did hatch and I thought all was well. It was up scratching in the coop with her. That was three days ago. This morning, I got up and the silkie was outside by herself, and I knew that was a bad sign. Furthermore, I didn't hear the chick. Sure enough, when I got there, it had died. It was still warm, and I held it for a while to warm it up. But its heart wasn't beating and it was clearly gone.
Anyway, just my anecdotal evidence that a young silkie who doesn't show committment is probably going to be more of a frustration than not.