Suppose a car has momentum of 1000. It crashes into a tree and stops. What's its new momentum? How much did it lose?

its new momentum is 0 because it lost 1000

unless you want to get technical about it.
because its sitting on the earth , the earth is rotating around the sun at apx 66,000 mph
and who knows how fast our solar system is rotating around our galaxy , and who konws how fast our galaxy is rotating around our universe , etc...

but if its stopped on the earth then in reference to the earth its momentum is zero.
and it lost 1000.

Im not sure where your going with this unless your going to
try again to show that the concept is invalid using only 1 or 2 masses.

because if you stop one of the masses that has been accelerated at 5 m/s then you can subtract all the force that
was applied to accelerate the mass at the end by completely stopping the mass.

but if your going to stop 1 at one end then you must also stop 1 at the other end and this will
cancel out each -momentum with a +momentum.

even if you do that , there are still 20 masses being accelerated constantly at 5 m/s/s applying a force to the pipe.

but try as you may.

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