What next?

LOL , ok...

-2 because it's reversing not just stopping.

were not dealing with a free energy device here.
so you arent allowed to add free energy.

and the turnaround will not add any energy to the mass.

the turnaround does not supply a force to the mass it is like a curve in a road that your car travels on , I cant remember having to press the gas pedal to the floor board just to get through a curve , LOL.

I usually take my foot off the gas. LOL

the mass already has enought momentum to clear the turnaround.

and the most negative momentum you could possibly count
would be the mass * the masses velocity.
which is the masses momentum.

so whatever the single masses momentum is is the most
you can use as a negative momentum.

and the opposite occurs at the other end in the other turnaround , so once again I state that the two masses
momentums in the turnarounds cancel each other out.

the only remaining force is the force used to constantly accelerate the 20 masses.

we havent calculated the force to the pipe due to accelerating the 20 masses "constantly" so we cant use
your +20.

the number will be much greater than that.
to accelerate a 100 kg mass from 0 m/s to 5 m/s
requires more force that to accelerate that same mass
from 0 m/s to 1m/s.

until we know the amount of force applied to the pipe
due to accelerating the "20" masses constantly we will not be able to arrive at a close number.

note: constantly...

and by the time each of the "20 " masses gets to the end of the pipe and enters the turnaround its velocity might be 30-40 m/s faster than when it exited the turnaround at the other end.

so we cant determine the validity of the concept using your

3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.