An unused (yet) enum type cannot display the enum ranges. An empty table containing that type cannot display enum ranges.
Yes, it can.
CREATE TYPE rainbow AS enum ('red','orange','yellow','blue','purple');
SELECT enum_range(null::rainbow);
enum_range
{red,orange,yellow,blue,purple}
I get the distinction between classes and objects. But in many cases, like this one, you need to obtain an instance of a class - a null is generally sufficient - and pass that instance to a function. The function can then use "pg_typeof(instance_value)::oid" to derive the oid for the corresponding class. This is a common idiom in PostgreSQL.
The only improvement, besides the error handling point, that I see to be had here is your understanding of how the system works.