Switching replication master in cascading replication scenario
Yes, it's safe. B is an identical copy of A that must always be at or behind it in time, so it is safe to switch replication from B to A.

Personally, though, I'd use WAL shipping from A as my fallback. I'd leave streaming replication configured to use B. If it's unreachable, C will try the WAL archives, and it'll find the WAL archives from A and replay from them, periodically retrying B until it can connect again.