What exactly the rest of the group was/wasn't there for is made deeply ambiguous by the fact that most of the relevant characters vanish into the Recruit Zone, but I think there's a reasonably straightforward reading that at least one of Delita, Agrias, or Mustadio has been present for every battle in the game so far? Nobody's been there for everything but you can get it down to a pretty small minimum number for full coverage, even disregarding the generics.
Also, yeah, this. Insofar as we apply the Durai Papers framing device to the entire contents of the game, it's entirely impossible for them to exclusively consist of a secondhand recounting of any one individual's experiences. And once we allow them to incorporate synthesis and/or extrapolation, it's extremely easy to construct theories of their origin that don't depend on any given person providing testimony.