Compton scattering with a space-like separation for the absorption and emission points are a well known phenomenon in quantum field theory. Classically these events would be causally disconnected, but in a QFT the propagator is non-zero and instead has an exponential suppression in the space-like interval.
Still, we were originally talking about gravitons. "Exponential suppression in the space-like interval" means that it cannot apply to gravitons (or at least, it cannot apply to gravitons on astronomical scales).
So, backtracking the argument a ways: Do you have any examples of the numbers of particles being different in different frames or coordinate systems that works at all distance scales?
What makes you think this cant apply to gravitons, or more generally, what makes you think that gravitons are relevant over large distance scales?
> Do you have any examples of the numbers of particles being different in different frames or coordinate systems that works at all distance scales?
Sure, it’s called the Unruh effect. Any accelerated frame (or observer in a gravitational field) will see a different particle vacuum than observers in a inertial frame.
This means that if you start in empty space and then accelerate, the space will suddenly not look so empty and will be at a higher temperature with more fluctuating particles.
This is a phenomenon with experimental consequences and uses:https://arxiv.org/abs/1301.3819 https://www.nature.com/articles/s41567-019-0774-3