I made something similar.

First I created an own UI Page which contains the Email Preview: <g:inline template="notification_display.xml"/>.

With buttons "Send"/"Cancel" which will trigger the event for sending the email.

Simple Ajax call for gs.eventQueue(..).

The UI Action Button opens the UI Page and set the following preferences for the email preview:

.setPreference('sysparm_notification_id', [notification_sys_id]);

.setPreference('sysparm_target_id', [target_sys_id] );