Colder air. Not gravities fault, it's never gravities fault. Gravity is a female.

Just adjust the hop-up until it feels right

a LaGrange point...where the forces are balanced and cancel each other out.

Under STP and zero wind, a bullet when fired from a barrel it cannot rise above the y-axis of the barrel.

Well, the universal gravitational constant is, well a constant.  everything else is the based on the masses and the square of the distance.

F = G * ((M1 * M2)/r^2)

There is no such thing as bullet rise.  As long as you are on Earth, the trajectory of a bullet will never rise with respect to Earth.  A bullet's trajectory may start upwards because the barrel is pointing upwards, but that trajectory will never rise.  That's why bullet trajectories are always - ALWAYS - drawn as an arc with downward concavity.

Now, reduced gravity can affect the maximum height that a bullet achieves during its travel.  Assuming you have top-mounted sights and have them zeroed at some distance, that means that the barrel is actually pointing slightly upward to account for the bullet drop due to gravity.  Imagine you want to throw a baseball from right field to home plate.  You don't throw it in a straight line.  You throw it upwards in an arc.  If gravity were suddenly reduced, i.e. you were standing on the moon, it will take longer for the bullet to level out.  All of a sudden your 200 yard zero becomes a 600 yard zero and the peak of the trajectory will be higher with respect to the ground.  The opposite would be true if you zeroed a rifle on the moon and then took it down to earth.  The zero would be much closer and the peack of the trajectory would be much lower.  In either case, the trajectory will still start to drop, albeit at a lower rate, the second the bullet leaves the barrel.

To the OPs question - there is no "bullet rise" calculator because nobody cares how much a bullet will rise with respect to the earth.  Seriously, does it matter if the peak of the trajectory is 6" or 10" above your target as long as it ends up where it is supposed to at the end of it's travel?  All people care about is how much it will drop with respect to their zero over a given distance - which is exactly what a bullet drop calculator calculates.  Also, bullet drop calculators don't factor gravitational variations since they are too small to be relevant in any practical sense.

The forces relative to the planet and the star... but the planet is being pulled in orbit around its star due to (you guessed it) gravity.

As the bullet goes on a level path, the curvature of the earth makes it look like it's rising.
Sheesh, it's simple.

All of your "scientific" diagrams are based around a flat earth. I expected better