This "what if" game is pretty much inviting people to come up with the worst scenarios that they can think of. Recent news has many people's imaginations running wild. It's understandable, but we try to keep the forum free of conspiracy theories and discussion of situations where we come up with scenarios of us vs, essentially, our imagination. In these situations, we always lose. For this reason, this thread will remain locked.
I know very little about Session, but as far as I know, there's no evidence of backdoors in any of the messengers mentioned above.
Signal supports reproducible builds: https://github.com/signalapp/Signal-Android/blob/main/reproducible-builds/README.md
Molly supports reproducible builds: https://github.com/mollyim/mollyim-android/blob/main/reproducible-builds/README.md
SimpleX, if I remember correctly, can't support reproducible builds because of Haskell for some reason. Hopefully they find a way to allow their users to verify their apps in the future. They state in their privacy policy that server and client code aren't modified. I've seen their promises being referred to as "legally binding," but I'm not a lawyer so I can't say whether that's accurate or not. I'd say that if you have any concerns about SimpleX, they have a pretty active group called "SimpleX users group" in SimpleX where the lead developer, other members of their team, and regular members of their community address many concerns people might have.