The oldest surviving non-fragmentary copies of the Septuagint do include these extra books. Whether earlier copies, that don't survive, included them or not, can only be speculation. But you raised this issue as a "valid reason" for rejecting those extra books ("There is no evidence that they were included in the original versions of the Septuagint"). If we can't know and can only speculate, how can that be a "valid reason" for rejecting anything?
It's definitely a less significant point, but the reason I brought it up is that many make the argument that because they were included as part of the Septuagint, then they must be part of the Canon. Which is ridiculous on its face because they were never included in any Hebrew-based canon.