Originally Posted by
Altima of the Gates
Sakura was activated as the grail early on due to more than love about Shirou. The Shadow can't come into being before it is connected to her.
The second option is even more dubious. Why would it sacrifice it's host before it was born? That was the whole reason why it put her back together after she was torn apart by Gil (only the outside, but still put her back together).
The next thing about the attack on the temple. If Zouken was going to attack and use Kojiro's body as the vessel for Hassan, why would he not have bothered to trick Medea as he did in canon and lay claim to the temple, instead seeking to escape?
Finally, "Sakura just goes yandere because x, and things happen" is kind of, not to be rude, a little lazy. To be clearer, it's not something easy to just expedite plot elements with "villain in a can". It flies in the face of her character.
However, I'd consider letting the second point go just on the fact we don't know the details on how Zouken trolled be in canon anyway.