If the doctor never existed, River would never have been born, and her journal wouldn't end up at the wedding. It looked like River gave it to Amy, how?

Rivers future self could have gave it to Amy.

I can't really remember that far back, but maybe it wasn't her actual diary- maybe just a book that looked the same but not written in?

when te fall into cracks te were never born, but all the things te did are still there, the doctor was trapped on the other side and he gave the diary to river, it was blank because she had no memory of him o it got erased

te have to consider the amount of times River has travelled throughout her parents' and the doctor's time streams. If te really think about how much she bounces around the time vortex, it's quite possible, indeed, that a future version of herself gave it to Amy.

When someone starts asking domande about Doctor Who time line, that when it's gets all complicated.

Well, as I know, it's just complicated. As the 10th Doctor detto it's all "Wibly Wybly, Timy Wimy Stuff"(Forgive me I don't know how to write it but i think te understand).
But I have one other explaination, it's River from the future, who was born in a time Amy can't reminber, who came at the wedding which is in her past, being told da the future version of the doctor(the one at the wedding) that this was happening o will happen o is happening. It's complicated.