moving_target: Newton’s second law, as he wrote it in his Principia, is: “LAW II. The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
If any force generates a motion, …”
Nowhere in the principia did he invoke the ideas of ‘net force,’ ‘zero net force,’ or ‘non-zero net force.’ The key is the phrase ‘motive force.’ If a body is restrained from movement and a force acts on it there will be two equal and opposite stationary forces. If a body is not restrained from movement there will be two equal and opposite moving forces: the motion producing force and the inertial force opposing it. If an object capable of generating a force met no opposition it could not even generate a force. Furthermore, if a body did not produce an opposing inertial force, any applied infinitesimal force would give it infinite acceleration. The inertial force met by a motive force is why the acceleration is in proportion to the motive force.

As for Newton’s third law, he continued immediately after it with explanatory text that begins with “Whatever draws or presses another is as much drawn or pressed by that other.” In the law and its explanations he made no qualifications and put no restrictions on the sources or natures of the forces involved.

Regarding centrifugal force you wrote Anything that is not real is imaginary, hence beyond objective measurement, and therefore not a subject of science.