it's density that makes things fall, not the theory of gravity.
The theory doesn't make things fall, that is true, but the fundamental interaction gravity does. It's the observation of this fundamental interaction that gives rise to the theories explaining how this interaction behaves.
Here's a nice demonstration of objects of different densities falling in air and then in vacuum.
The feather falls slower in air both due to being so light and having a shape that creates a lot of air resistance. The feather will also be more bouyant in air than the bowling ball due to lower density, although bouyancy is a rather negligible force here. Without the air to exert any force on the objects they fall at the same rate, regardless of density where a density of 0 is vacuum itself.
A balloon filled with helium is less dense than the air at Earth's surface, so it rises. In a vacuum chamber such as the one in the video a balloon (that would keep its size in vacuum) would hit the ground at the same time a bowling ball or an anvil or whatever you can think of. Density says fvck all about how things fall unless they're falling through a fluid which itself has a density.
An egg floats in salt water for the same reason helium balloons float up in air, it's less dense than the fluid they're in. The egg sinks in fresh water because fresh water is less dense than water with salt dissolved into it. How you think this proves anything regarding gravity I don't know, but it's a great demonstration of density and bouyancy.
Remove "gravity" from the buoyancy equation & it will STILL WORK PERFECTLY FINE!
Well, yeah, if you're changing velocity (accelerating or decelerating). If nothing is moving due to no force such as gravity acting upon it then it's just stuff floating and density is irrelevant (unless the object is massive enough to have its own significant gravitational pull).
Regarding velocity change and bouyancy, think of stuff in a car. If you slam the breaks then every loose object will come flying towards the front, or will it? If you have a helium balloon in the car it will go flying towards the back, but you can really think of it as the air being denser than the helium so the air goes flying towards the front, leaving no space for the balloon in the front, thus displacing it to the back.
Same sh!t would happen inside a vessel in vacuum, except
everything, including the helium balloon, would go flying to the front of the vessel at the same speed regardless of density.
Bouyancy is only relevant for objects in a fluid which has a density under the influence of a force such as gravity or acceleration. Vacuum has no density and thus no bouyancy, so objects are only affected by bouyancy if there's gravity or something otherwise accelerating them in a fluid such as air or water.
I'll happily educate you further in the world of physics if you want to.