Positrons don’t move backwards in time in any physical sense. Their mathematical representation is like an electron with a negative time coordinate but this is physically dubious because it fails in curved spacetimes. Electron-positron pairs are entangled because they are correlated by the process that created the pair. Entanglement is just a correlation, not some magical link.
Entanglement is more than just a classical correlation though - it’s specifically the kind where the quantum states can’t be described independently, which is why Bell’s inequalites get violated and why Einstein called it “spooky action at a distanc”.
It’s stronger than a classical one, yes. But Einstein’s description is mocking the idea of entanglement as an active link that allows distant states to be changed by local actions.
If an electron were to become exposed to a positron in a manner within which they could become entangled, they would annihilate. The resultant photons would be entangled, but their respective energy/momentum values would depend on the incoming electron/positron momentum/energy values.