Server 2003 R2 Schema Update
1) Schema updates are not related to licensing in any way, so, yes, you can run a schema update from ANY Windows installation media (probably excluding SBS, but that's a completely different product).
2) No, SP2 is not a prerequisite for this schema update.
3) No, in order to fully restore AD from a backup, you only need to restart a single DC in DSRM and perform an authoritative restore; then standard AD replication will take care of overwriting all other DC's copies of the directory database with the one you restored (that's why it's called "authoritative").