You're right, it doesn't show that Maria left then. However, it never shows Maria say a single word, she doesn't have a single word said to her, and it doesn't show her picture. And before this scene, Beatrice has already handed Maria the letter, and she jumped around happily, which could have been a sign that she was leaving.
The scene with the ring falling should have happened without time stopped. I don't think Maria would see Ronove and not even say hi, assuming time was moving and she was there. If she couldn't see Ronove, she would have asked Beatrice who she was talking to.
I guess it doesn't prove outright that the seal is a fake, but it is pretty weird. Why would she give Maria the letter before getting the ring? That much obviously happened.
The only question is whether Maria stood there silently as a Great Demon had a nice chat with Beatrice before disappearing with her into Battler-land
Oh, and then Beatrice would have to rematerialize and chase Maria down, unless she sat there without even being told to, until Beatrice came back...