and yet, atmospheric particles that have a mean free path that doesn’t intersect with other atmospheric particles, or when they do the average velocity delta and direction reach escape velocity, escape and are lost.
You might want to rethink that. It’s a useful model in bulk in the lower atmosphere, but it’s far from true in the upper atmosphere.
First, while the upper atmosphere is much less dense than the lower, and the fluid approximation becomes less and less useful as you gain altitude, that still doesn't mean that "a bunch of particles in free-fall orbits" becomes a useful model. The average thermal velocity of a molecule in the upper atmosphere is still well short of orbital velocity at that altitude. Some molecules acquire sufficient velocity to escape, sure, but that doesn't mean the others are in orbit.
Second, the Earth's atmosphere is not a good analogy for what is happening at the heliopause anyway.
