I already think it's strange that they decided to put Rum in the movie, even if all background because it's so early... but I understand why when it comes to hyping up the movie. Heterochromatic woman is a prime example of this. They utilized "Rum" and the fact that they have a prosthetic eye to their advantage (and I know they aren't the same thing, but you'd be surprised at how many people still think she's going to turn out to be Rum, or at the very least question it).
Even 1 year later, we may still not know Rum's identity in the manga. I think with a movie "reveal" like 18, this was done simply because we had plenty of information to piece together Okiya=Akai in both the anime and the manga. It did not matter at that point.
At this point, there's still not much to go in in the manga, especially the anime... so I think taking anything as evidence (especially something that can hardly be regarded as any form of proof) in a movie not connected to the manga is a bit presumptuous.
I myself am still betting on a new character. There's really no reason for me to think otherwise at this point.
Here's some major spoilers from the Red Thread's page
(Do not open - If you wish to remain unspoiled)