This directly contradicts your prior statement, in which there is one inertial FoR in which that applies for a given moment. This is the usual problem with your claims - you keep disagreeing with yourself.
No, it doesn’t contradict. What I said that for any given moment, for any infinitesimal duration, the “relative” acceleration would be 9.81 m/s2. That means it is always 9.81 m/s2. Pick whatever moment you want in any inertial frame you want…its always going to be 9.81 m/s2. One inertial frame may apply for that moment, but in the next moment, in the next inertial frame and in every moment and ever inertial frame after that it will 9.81 m/s2.
You are once again (pretending to be) hopelessly confused. The Earth's acceleration cannot be "considered to be proper acceleration" or not. It can be expressed as one or the other.
It isn’t an either/or proposition. Proper acceleration can be expressed as relative…but doing that doesn’t change proper acceleration into relative acceleration or mean that proper acceleration no longer applies.
Proper acceleration by definition cannot be relative. It is “the physical acceleration experienced by an object”. Relative means “considered in relation or in proportion to something else”. What is being physically experienced isn’t considered in relation to something else, whether its acceleration or pain. You don’t have to ask anybody else if you have a toothache. It doesn't matter if someone else doesn't believe your tooth is hurting. You know “objectively” if your tooth hurts or not. There is nothing subjective about it.
If you are accelerating in a car at 100mph, you will experience physical effects from that. You will feel it. Without ever looking outside, you can “objectively” know you are accelerating. It doesn’t matter if your “relative” acceleration is 5mph to the guy on the sidewalk. If you hit a brick wall you will still die. His perception of how fast you were going doesn’t mean squat. Do you deny that is true? Do you not understand that what is physically perceived from one frame doesn't effect what is physically occurring in another?
You can’t have it both ways. Either the earth is constantly, physically accelerating at 9.81 m/s2 or not. For the sake of argument, imagine someone else in an inertial frame in a galaxy far far away perceives earth’s “relative” acceleration as 7.2 m/s2. Does that mean “gravity” is weaker on earth? What if someone else in another inertial frame perceives it at 12 m/s2? Is" gravity" stronger?
If there is an infinite number of inertial frames and the earth’s proper acceleration can be perceived at an infinite number of different rates…how can FET say it is accelerating at any specific one if it can't be objectively determined? For that matter, how can FET even say that it is accelerating at all, if you can’t determine it objectively? Has any flat earther ever observed the earth from an external inertial frame?