Boslandew,
Your answer sounds plausible.
The 'tradition' is older than the Korean war, but it may well be one of the reasons it persisted!
The subject has already been done to death elsewhere.
But in very brief... in the very early and small helicopters, with only one central collective, it makes more sense for the average right-handed person to fly right hand on cyclic and left hand on collective, hence from the right-hand seat. The 'tradition' has persisted, even when dual collectives became the norm.
For fixed-wing, I've never been sure what was the chicken and what was the egg.... pilot/driver on the left because that was already the habit in cars, hence left-hand circuits and flying to the right of the railway line... or the other way around.