I guess the error of 10-15 is due to instrumental errors and general relativity not being the final answer.

That is indeed the precision that is reached with the experimental setup. Nothing more nothing less. This is also why MICROSCOPE will allow for an improvement of a factor of 10 in precision but not more. The apparatus itself at some point limits the accuracy of the measurements.

The equivalence principle doesn't hold in the quantum realm. Please take a look at this

This is exactly what I have said in some of the other comments. As soon as you try to unify the four fundamental interactions, you predict deviations from the equivalence principles. However, those deviations are much smaller than 10^-15 level.

Also, there have been experiments suggesting the deviation of the classical equivalence principle.Here

True, and the problem was not removed after a more recent and accurate experiment happened (see here). There are other tests still going on to understand that. Neutrons are strange beasts :)

Edit: regardless of where i am on the globe, i am not buying the snow-only snow globe :)

Do you know I hesitated in linking a flat earth image... But this would give them too much free advertisement :D