Dear @MARUBUN SUPPORT
A1) You can use Java version 8.0.482.8 for SiteMinder 12.9, Please refer the vendor specific documentation for specific improvements or fixes of Java.
Broadcom provide the Reasonable Commercial Effort for change of Java in Admin UI, Please refer the 12.9 Platform Support Matrix and also a NOTE in Resolution section in the KB#235267
A2) It is fine to stop the Policy Server service before the upgrade from 12.8 to 12.9, We introduced the step due to the Upgrade Readiness utility in SiteMinder 12.6 which primarily checks for unsupported 32-bit binaries, missing libraries, and custom authentication schemes that might fail in the 64-bit SiteMinder environment.
A3) It is recommended to check the installation logs for each component, The following also helps.
For Policy Server, Check smps.log for successful initialization and ensure no "max connection exceeded" errors are present.
For Policy Store, Run XPSSweeper to perform integrity checks. Confirm that the store schema has been successfully upgraded to the target version
For Admin UI: Log in to the Administrative UI and perform basic tasks, such as viewing a policy or creating a test object, to verify functionality,Verify the AdminUI server starts without errors in the Wildfly logs.
For Access Gateway: Check the server.log and Apache/Tomcat logs for successful startup and request processing.
We would recommend to test the upgrade in your lower environment before production upgrade.
Thank you.