I am a bit curious about how you determined
1. Law of cartoon motion
Objects in flight stay at the same elevation and velocity until they run out of momentum, at which point they fall straight down. This may change with soon-to-come updates (parabolic minecart paths).
In the real world, launched objects will travel on parabolic paths. I realize they did not in this version, but it would be reasonable that Toady might try to approximate a parabolic path as moving an appropriate distance and then falling down.
Anyway,
if an object travels on a parabolic path, then the distance it travels will be proportional to its energy, not its momentum. Here is a proof:
Assume an object is launched with velocity v at angle a, and ignore air resistance. The time the object spends in the air will be proportional to the y component of the velocity
g*t = v cos(a),
where g is the acceleration due to gravity. The distance traveled will be
x = t *v sin(a) = v2sin(a)cos(a) / g
which is proportional to the kinetic energy (1/2 mv2).
Toady has a Ph.D. in math, and could certainly work this out (although I have a Ph.D. in physics and it's not immediately obvious to me without thinking about it, so he may not have). If he wanted to produce such physics without parabolic paths, he could just say that a constant force is applied to objects in flight, and they fall to the ground when they run out of energy. Then the length they travel would be
x=E/F, where
E is the energy and
F is the force. If
F were a constant which doesn't depend on object size or anything else, then you would have projectile distance proportional to energy, as you would expect in the real world, without the messiness of coding parabolic paths.
All this is a very long-winded and technical way of saying that it is not valid just to assume that pre-minecart projectiles fly a distance proportional to their momentum, as it would be just as easy (and more realistic) for Toady to make it proportional to their kinetic energy. That being said, it is quite possible that it was proportional to momentum, and I would be very interested to see proof of this.