Apparently, the brothers had just recorded evidence that a manned Soviet spacecraft somehow got off course and left Earth's orbit, permanently. </quote>
This single sentence is enough to discredit the whole story.
It is technically impossible for manned spacecraft to leave Earth's gravity by accident.
It's unlikely, but far from impossible. The delta v to get to low-earth orbit is about 10 km/s (more depending on drag), while the delta vs from low earth orbit to the Lagrangian points, low lunar orbit, or even earth escape velocity are of the order of just 3-4 km/s.
This single sentence is enough to discredit the whole story.
It is technically impossible for manned spacecraft to leave Earth's gravity by accident.