When a point is disabled, scripts which attempt to use the points should error and failure to return values. If it is a meta point, it shouldn't get a value, if it is a point link it shouldn't set the target and if it is a Scripting data source it may exit the script right there.
When communication is lost, the point may report unreliability on other pages of the system, but I do not believe that is currently available in scripting contexts. I will mention the possibility of adding it in future versions. For now, you can use logic on the timestamp to determine if communication was sufficiently interrupted to change the behavior.
I do not know what you mean "offline / online"
There is an open github issue about changing the "return;" behavior to not produce a value, but as it is right now the only way I know of returning from a script and not producing a value is a type result error (i.e. return "DEFINITELY NOT NUMERIC!";)
If you meta point is on context update, it would only be executing when new data was available for its source point(s).