The Mirrorball Man wrote:
There is no "before". The Prophets are not linear.
Indeed. There is no contadiction if you avoid looking at it linearly.
"Emissary" was the initial point of contact between the Prophets and Corporeal Entities. They were not aware of Sisko before that point. But from that point of insertion into their existence, the awareness spread throughout their existence, in a way that from our perspective would appear to be both backwards and forwards in time. From their perspective, that's just their natural way of being.
Once they had encountered Sisko in "Emissary," it was as easy as us walking down the street for them to go back in time (indeed, they wouldn't even understand the concept of going back in time, because they are present in all times and none) and create the person they needed him to be as revealed in "Shadows and Symbols."
As Sisko describes it in Warpath
- "They didn't know until they'd met me that they needed to create me."
As for how the Prophets or Sisko could not know this beforehand if they went back in time from our linear perspective, that is also not inconsistent if you take "Accession" into account. In that episode, Akorem Laan in the original timeline had left his masterpiece poem unfinished. When the Prophets put him back into the timelines where he came from, the poem then became finished. But Kira remembered the old version where the poem had not been finished, even though it now had been.
All of which means that, whatever method the Prophets have of changing events in the past, it doesn't completely wipe out the original chain of events for the entire galaxy so that they never happened. It only blends in, tweaking the bits that concern them and leaving everything else untouched. That is why the Prophets had never encountered Corporeal Entities until they met Sisko, at which point they had always known about them.