It's hard to tell if momentum is being conserved or not with so many particle, but in the example you mention there is one particle moving to the right, which i

It's hard to tell if momentum is being conserved or not with so many particle, but in the example you mention there is one particle moving to the right, which is the OMFG. So to make the test simpler, let's start with a single particle. This particle is initially at rest, just as with the protodisk, and nothing happens if we wait for a while. Adding in another particle of the same mass, we see the two particles move towards each other and meet in the middle, implying momentum is conserved. So I am thinking momentum is probably conserved for all cases, to some degree of accuracy