I contacted WB Games to get a definitive answer to the Harley Quinn situation but they just replied "we do not have this type of knowledge inside of the department".
Great, it's absolutely ridiculous how WB is handling this whole pre-order DLC thing. There is Harley Quinn, Batmobile Skins, Red Hood, Prototype Batmobile, Wayne Tech Booster Pack, Costume Skins, etc. and it is impossible to receive them all, even if you are willing to pay for the most expensive version available. They are either platform or retailer specific. WB have really taken this to a new level.
Btw, this is how the Premium Edition is advertised on xbox: http://tinyurl.com/ntyxgyo
Notice the "Harley Quinn Story Pack" that is included? Why should it be any other way on PC?
However, since you got confirmation from WB I'll have to cancel my pre-order with you and buy at another digital retailer who is offering the Premium Edition with Harley Quinn included. Sorry for that, but there is only Warner to blame.