Elaborate on the "Christa survived" part, because it's pretty clear that she died.
If Clementine distracts the scavengers she throws a rock at Winston and yells for Christa to run away. If Clementine sneaks away, Winston sees her and stabs Christa in the leg with a spear and Christa yells for Clementine to run away.
It was most likely she had a miscarriage because Christa would've protected the Baby no matter what, so it was either StillBorn or Miss carried. I think the baby died from hypothermia. Because Kenny was angry the moment he found out the baby was dead.
The mirror shard was a gift from Sirius given to Harry in “Order of the Phoenix.” The mirror is part of a set that allows people to communicate with each other. After Sirius' death, Harry put it in his school trunk and forgot about it.
The Firebolt is a costly broom and Harry Potter was among the first to own one.