Here is my parital theory:
The merchant ship was operating on auto-pilot, with all humans sound asleep. The collision happened at the sharp right-hand turn at the beginning of the “odd manuvering” not the end. The humans woke up and spent a few minutes wandering what the hell happened while the auto-pilot is speeding up and turning back toward their course.
The humans then disengage the autopilot, slow down, make the U-turn and after they confirm what happened they call in the collision. They stay a while to render aid (the meandering part after the U-turn), then resume their course to Japan.
This theory explains the odd track, the delay in reporting the collision, and why we have almost no information about the collision itself. It does not explain why no one on the destroyer were able to see/react to the cargo ship.
That would never occur; i.e., all humans asleep. Certainly not in a busy sea lane around Japan. An auto-pilot on a ship maintains course and speed; it doesn't avoid obstacles.