Did you try to change the schema using something like this?
p_enabled => TRUE,
p_schema => 'Your_schema',
p_url_mapping_type => 'BASE_PATH',
p_url_mapping_pattern => 'api',
p_auto_rest_auth => FALSE);
This enables a schema. I can do this but the REST call definition uses the default schema (we think it is the first provisioned schema) which is shown when the export script is examined.
We want to create a number of REST services that use different schemas.
This appears to be resolved with APEX 18.2 - allows us to select an associated workspace schema and install the restful ORDS objects on a schema by schema basis. The schema name can then be aliased to reference within the REST URL.
So it looks like we have to upgrade!