OK, well lemme bring some doubt to your minds.
First, let me say that I did just watch the movie but a few days ago. It's fresh in my mind and I've noticed a few little things that could hint at it being Bianca.
1. When Ash and Co. drive up to Lorenzo and say goodbye, they then ask where Bianca is, to which he replies that she might have just left for the market, and they might have just missed her. It then pans up to Bianca's room, showing that she left her hat hanging on a chair. If Bianca was going to the market, she most definitely would have brought her hat with her. Since she left her hat that means she was home. I believe she left her hat on purpose, AND did not speak on purpose, to make Ash believe that she was Latias, perhaps to just avoid any awkwardness or embarrassment.
2. Why would Bianca have drawn such a picture anyway? Bianca was shown to be an artist (and it definitely was not Latias who drew it). For her to draw such a picture depicting Ash and Pikachu, she must have taken some sort of liking to him, romantic or not.
3. This is too inconsistent to be legitimate proof( and my memory is also a bit hazy), but throughout the movie Latias (disguised as Bianca) USUALLY runs by swinging her arms outwardly (there are times when does NOT do this, though). At the end of the movie, Bianca or Latias is seen running to catch up to the boat by swinging her arms forward (like a jogger or a sprinter). After kissing Ash, she also runs away using this running method, leaving it to believe that it was Bianca.
Now, I know it would make a lot more sense for it to be Latias. She took a great liking to Ash and the two had a fantastic bond in the short time they had together. However, Bianca and Ash still had some friendly interaction, and Ash made great heroic efforts to save her and the city. An attraction could have easily have been there.
With that said, I'm pretty sure it was Bianca. No guarantees, as it was meant to be ambiguous. However, I'd very much like to ask a staff member of the movie's production if anyone knows where I could contact them.