Voltage generated by piezoelectric effect
The voltage is related to the amount of mechanical strain/stress in the material and is a dynamic equation (has states changes over time):

Source: https://onscale.com/piezoelectricity/the-behavior-laws-of-piezoelectricity/
In short: to calculate the voltage generated, one needs to know

the material and it's parameters
the stress in the material (and how that is changing over time) which is derived from force and material properties.


When the piece of insulator returns to its initial state a voltage is
generated by which is its polarity compared to the original
voltage(step 1)?

Without getting too deep into the equations, the stress in the material starts to generate an electric field (the electrons have to be there in the first place). This then generates a voltage (and reduces the strain). If the strain is held steady then the voltage will be steady ( they build strain gauges with piezo electric materials). As the material relaxes, the voltage of the material falls (zero strain = zero voltage). So if the material is relaxing the voltage doesn't reverse, it falls toward zero. If you wanted to get a negative voltage, then you'd need to pull the piezo electric module instead of push it.