time travel - Why didn't River know what would happen to The Doctor at Lake Silencio?

I'm talking about the Season 6 finale "The Wedding of River Song", about this moment:

I'm specifically talking about the moment

when River is inside the astronaut's suit, not about the River that watches the whole scene from where the photo is taken.

_{(I won't bother with spoiler boxes from now, because this won't show up in the preview)}

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Okay, so River and The Doctor always meet in reverse order. The Doctor's future is River's past and The Doctor's past is River's future. She has already fallen in love with him and obviously knows him well. That means, she remembers The Doctor from her past. Again, her past is The Doctor's future.

If The Doctor has a future, and what happens at Lake Silencio is a fixed event in time, The Doctor cannot die there. She is really clever and quite used to this sort of 4-dimensional thinking, so it shouldn't be hard for her to put one and one together.

So how about that? How come she doesn't realise, that "shooting" him there will not cannot hurt him?
Note: She just learns at her wedding, just before going back to the Lake, that this is not the real Doctor.