You’re right. Someone who was present for all of the Freighter/Falcon stuff was a major leak.
The shot of Maz handing someone (previously presumed Leia) the lightsaber wasn’t in the film, was it? They used a different shot, and it was Rey. Makes sense as I’m sure they were no where near finished with Maz and needed something that obscured her. Still, why show it during ‘my sister has it’?
Why was the voice over used at all? “The force is strong in my family… you have that power too?” In the context of the film and what is explicitly stated, Kylo Ren’s the only one that fits the bill. But Rey’s the new hope. So that’s one of the main reasons (in addition to a few solid hints throughout the film) that I think Rey is most likely Luke’s daughter.