Whether this POD could plausibly have affected the Schlewsig-Holstein situation at all, too, I'm not entirely certain... but I suspect that Denmark getting all of Schleswig back (rather than just its northern half) after WW1 would probably be the limit of any added benefits to Denmark.
Denmark was actually offered to regain ALL of Schleswig-Holstein after WW1 by the Entente. Denmark refused and instead pushed for a referendum.