Can the approval be prevented from editing (even the state stage) once the cumulative state is moved to Approved/Rejected state
Accepted answer
As far as I can tell, there is nothing built into RTC that prevents an approver to change the approval state which impacts the cumulative state. An approval is also not an attribute so you can not set it to read only in states.
You would have to create a custom advisor. That is a Java Server extension that prevents work item save for a special condition. See https://rsjazz.wordpress.com/2014/05/26/only-owner-can-close-workitem-advisor/ as an example. Follow the link to the enablement material if you want to implement something yourself.