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1) You can find answer in the Data Guard doc http://download.oracle.com/docs/cd/E11882_01/server.112/e10700/log_transport.htm#BABCBEGJ:
Oracle recommends that a standby redo log be created on the primary database in a Data Guard configuration so that it is immediately ready to receive redo data following a switchover to the standby role.
2) I don't know but I'm sure Dr. Paranoid knows and will answer when reading your question ...
1. Standby redo logs are used only when the database_role is standby, it is recommended to be added in primary also so that they can be used on role reversal, however during normal working standby redo logs will not be used at all on primary.
2. In case of 3 online redo log groups, it is recommended to use 4 standby redo log group this is in case if log switching is happening frequently on primary and all 3 standby redo logs are still not completely archived on the standby and 4th can be used here as there will be some delay on standby due to network or slowness of arch on standby.
Use of the standby redo log groups depends on the redo generation rate, you can see only 2 standby redo logs are getting used while you have 4 standby redo log groups, when the redo generation rate is less.
So it is recommended to have one more standby redo log group when redo generation rate is high and all of the existing standby redo log group are getting used.