The cut scene where the voice of the absolute forces the party to its knees and SH's artifact drives it off. This happens just before the overland map is revealed.
We can get to the end of Act 1 really quickly, if we want; skip the grove, circumnavigate Moonhaven, then bypass goblin guards. Possibly start of mountain pass and Moonrise is Act 1 too?
It's difficult to read game's exact structure without playing the whole game, but I personally believe that what we have seen is most but not all of act1 and act1 will end at moonrise tower where nature of our tadpoles will be revealed, we will have to "commit" to our selection of companions and we will move to Baldur's Gate for act2 loosing access to act1 areas. Considering the similarities I am mostly comparing how D:OS2 was structured, though it is difficult to say at this time if BG3 will turn out to be D:OS2 2.0 through and through, or will they diverge as the game progresses.
But putting that aside, and going back to the point you made in the first place. Yeah, I do think you are right that the "control" cutscene will be likely modified and we will be shown the three, rather than have them just discribed to us. That would make their identity less of a mistery even without trailer spoilers.
