But it has also been mentioned several times that we're the only survivors?
The PC is the only survivor because they are the only person found alive at the blast site. It doesn't mean that there was nobody outside the blast radius that saw it happen.
The narrative is longer than just stumble upon Varric, add to party, meet Cassandra, add to party (or they other way around). You're not necessarily going to meet both Varric and Cassandra at the same time, but both of them are around the Temple of Sacred Ashes and they traveled together from Kirkwall to get there (and we now know why Cassandra was looking for Hawke too, she wanted to bring him at the Peace Conference).
We also know that Cassandra bring the Inquisition idea up at the Temple of Sacred Ashes...and that can only happen after she interrogated you (and Leliana going all "he's a sign of the Maker"...it's so going to happen).