All non-inertial frames are accelerating with respect to all inertial frames.
First point acceleration can mean not just a change in speed but also a change in direction. It is a vector quanity. So a child on a merry-go-round is experiencing constant acceleration which is at right angles to his direction of motion. (The acceleration points to the center of the rotation, while the motion is going forward)
True but in any accelerated field you can always tell what the acceleration is because it will produce a force that you can measure. That child on the merry-go-round will feel a force pushing him outwards to the outside of the merry go round. Now on a child’s ride it will be a light force but you could say get in a fighter jet and pull a tight turn and feel a very large force. In this case the jet is accelerating and i assure you that you can feel it. (OK I’ve only been up to about 2 g’s and it was in a very small prop aircraft)
Third you must understand that GR is at its heart a theory of Gravity, so in the Limit of low gravity it must agree with Newton, and it does.