There are two servers between which DataGuard is setup. The replication
happens from server-1 to server-2. server-1 is currently serving and
server-2 acts as a standby. I would like to know the following related
to this setup:
1) server-2 is in standby mode. Can server-2 start serving immediately
in case if server-1 goes out-of-service due to some problems? If not
immediate, how much time it will typically take for take-over?

2) Assume server-2 has started serving and hence it is in 'Active'
mode. Is it possible to do reverse replication of DB changes from
server-2 to server-1(similar to the replication from server-1 to
server-2 before takeover happened)?