What about "hvem dukker op der"? Is there a difference?
Nej, gammalsvenska, det finns ingen skillnad i mening. Ditt förslag är helt rätt! There is no difference in meaning your solution is correct. If we look for at tiny difference it is this: Your sentence is a bit less formal than the Duo sentence.
Clicking on appearing gives appearing as the danish word. Please fix.
Hvem kommer der til syne, is not acceptable?
No, it's a wrong order of words (though it would be understood), similar to 'who is there appearing?' - doesn't sound right