Istikhar,
1) HADR started at V8.2 (8.1 FP 7)
2) For DB2 LUW, there was no DB2 clustering prior to 8.2
3) No client reroute prior to 8.2
4,5) Prior to 8.2 failover was more or less manual. A secondary server would have been set up from a backup of the primary. Then the logs from the primary would be "shipped" to the secondary and applied there. This process could be automated. Then if a failure of the primary occurred, the secondary could be brought online manually by rolling forward the logs that had not been applied yet and changing the IP of the secondary to the primary.
HTH
Andy