Other observers to the chair experiment are not in another frame of reference. Everyone else observes the feeling of the earth pressing upwards against their feet while the experiment is occuring.
The earth accelerates upwards because that is what is observed and experienced. The words "observed and expenirienced" definitely do not apply to "graviton particles".
While those theories of gravity may be "possible," they are not emperical in the sense that they can be observed and experienced like the upwards movement of the earth. This is why the accelerating earth is the better explanation.
No, you have not observed the earth accelerating. That is a complete lie.
All you have observed is that if you step off something you hit the ground.
You see the ground rising to meet you. Everyone else sees you falling to the ground. And yes, they feel that the earth is exerting force on their feet, you can measure that force with a set of scales.
So that is the observation. There is a force acting on you.
The claim that the force is being produced by UA is a rationalization, you have not directly observed UA.
You can assert that UA is equivalent to gravity but it is still a rationalization, and you then have to explain the Cavendish experiment which is easily repeatable and shows that there genuinely is a force between objects.
Gravity is still not well understood and gravitons are theoretical. But you could say that about the "Dark Energy" which apparently powers UA. How does that work? Or Celestial Gravitation. How does that work? You accept all this quite happily because it fits in your world view yet you sneer at gravity despite it matching observations and being demonstrated by the Cavendish experiment. This is an example of the different level of evidence you require depending on whether something fits your flat earth model or not.