Balancing a stick using two different designs and comparing both
why feedback concept based on reducing effect of disturbance rather than reducing disturbance itself



It is difficult to measure the disturbance directly (exact value of force of the wind blowing on the vertical stick and gravitational force pulling the stick down). It easy to measure the effect (angle sensor in this example).
What we finally want to control is the angle in this case. not just the disturbance. Even if we perfectly cancelled the disturbance, the stick will still fall down if it had some initial velocity.



Is second model can be considered as feedback model or not?


System has force as input and angle as output. So the inverse system is effectively a (disturbance) force estimator of the original system. If the inverse model is not an exact inverse, the force estimate will not exactly cancel the actual force.
Yes, you are measuring the output, then using (what you think is) the inverse system, to estimate the force acting on the original system and you are feeding it back to cancel the disturbance force. But note that disturbance estimators are used; but not alone. They are used parallely with regular feedback. Integrator used in PID control is in fact a very crude form of disturbance estimator for many systems. It generates an estimate of slowly varying disturbances acting on the system.


Is both models are good to achieve desired result for this particular system


Not all systems can be inverted. Inverse of a system with a zero on the RHS will be unstable. Unstable inverse systems cannot be used in the feedback (to my knowledge).

4.which model(in above two)is in general preferred and why?

First method is usually used. See answer to #3 and #1 for reasons.

5.Or my assumption is wrong that for value of 'k' we need state model of G?

There are methods of calculating K where the system model G(s) is unknown. There are some auto-tuning methods used in process control where the gain is calculated in real time based on the system test inputs and corresponding outputs. I am not sure if those methods can be used for an unstable system like a vertical stick.