We seem to agree that the "Pacey must choose" thing is a little melodramatic and soapy - on its face.
And we also seem to agree that the whole thing is probably a red herring. It is a misdirection.
Where we disagree is that this misdirection is unimportant because it is misdirection. That's wrong. I think it is important.
There are literally thousands of ways the writers could have written the dialog between Sam and Nina. Thousands of ways they could have misdirected the audience. But they chose this one way - that the universe depends on Peter's choice between two women.
We the viewers are responsible, I think, for trying to understand why they chose this way as opposed to all the other ways, to misdirect us.
The reason we have this responsibility is that the show is going to spend time on this misdirection. A lot of time. The lie, if it is a lie, will have ramifications. It will control what the characters do with each other, how they behave, until such a time as they reveal that there is more to the story and Sam wasn't telling the whole truth. And because it will take up so much screen time we have to explain it, and deal with it.
Waving it away isn't going to work.
So, why did they do it? The answer is obvious. Olivia and Altlivia are versions of the same woman. One isn't better or worse than the other. Some sort of force is acting on the both of them so that they both are faced with making the same choices.
When Olivia finds out that her world depends on whether or not Peter chooses her, what will she do? Will she try to be the woman he wants? Will she sleep with him then? I don't know what she will do. But it is enough to know that her situation will become exactly like the situation Altlivia faced earlier this season, wherein wooing Peter is necessary in trying to save her world.
It is in the achievement of this parallel that explains why they are doing this.
I understand that very few people see the story this way, so I'm prepared to be completely wrong about all of this, YMMV.