No; this property is usefull when OPC is down. You had better enable it. Disabling that property will NOT disable for all domains.

You can still configure notification settings on WLSDM part; like ON/OFF backend notifications… etc.

The configuration is still same; all the functionalities on WLSDM are still supported by OPC. If you have a specific case; then please let us know in detail to reproduce it.

Regards…

Thanks for this wonderfull question !
Unlimited custom actions can be added on "Configuration > User Defined Metric Actions” page.

Here is a tutorial about adding custom actions for your reference: http://www.wlsdm.com/tutorials/advanced-weblogic-monitoring-develop-jmx-mbeans-and-automate-with-wlsdm-metric-actions.php