Is "Julemanden" idiomatic? Otherwise the definite form here, again, seems an overdo, as if there were multiple Santa Clauses (don't take this comment to that place, you know what I mean, we're all grown ups here)
It is idiomatic. It's like "The christmas man"
The definite article makes sense, because there are - presumably - numerous impostors of Santa, only out to scare children or give them their deserved gifts.
Is Duo accepting "Father Christmas"?