Well, I'd say she could get back if it was necessary. Like if she was to act as Rigsforstander or (let's not hope that!) her sister gets infected and is seriously ill.
QMII was once flown to and from Berleburg with an air force helicopter, I imagine a similar arrangement could be set up for Benedikte.
But apart from such an emergency, I doubt Benedikte will get back to DK anytime soon, even though she would no doubt be let through the border checks simply due to her status.