In the Benelux region I think Guillaume is moderately recognizable. I doubt the London society would find him so easy to know. The sorts of people that attend these parties are probably just as well versed in the "who's who" of royalty as we are.
In Luxembourg and the border region I think the HGD would be known on sight. A man on the street in Paris, London or Madrid probably wouldn't recognize Guillaume but anyone attending a party like this is "socially aware." They'd probably spot Guillaume across the room as easily as I would....like I said it was well known that Guillaume was present and when he arrived it was probably a big deal. In addition he was seated a the table of honor. If these girls didn't know who he was after all the fuss then Anna wasn't a very good spy.
Some people may find George's status as Grand Duke of Russia to be dubious given the dynastic disputes in the Romanov family. However, that George is royal is indisputable. His father is a Prussian Prince. George would at the very least be credited as His Royal Highness Prince George of Prussia. He doesn't use this title but it is his to wear. It is quite a sticking point between George and his father that he prefers the dubious title HIH Grand Duke of Russia. I would point out that many people are supporters of his mother's claim. I have no interest in current Romanov pretenders so I refrain from making a judgement myself. He also used to live in Luxembourg. It is possible that he and Guillaume are personally acquainted (not close personal friends). I suspect that George would have attempted to get to know the other royals living in Luxembourg while he was there.
I know all about German titles being names after 1919. It never fails when someone mentions a German title someone else feels the need to rehash the 1919 events. If anyone feels I may not be fully aware of that please know that I am...and spare us the routine.