Variations & pricing (Based on JCT Contract)

JCT Contract:

Architect issue AI for variation.
Main Contractor QS pricing the variation.

Measure on site or take off from drawings
Q1) Should the rate in the BQ be used to price?
Q2) Should that rate be enhanced? (Under same condition & same item????)
Q3) What should be included in the prelimnaries?
Q4) Should cost on working overtime for the variations be added?
Q5) Should cost for getting more labour on site to complete the variation work be included?
Q6) What is prolongation cost?
Q7) What is prelimnary thickening?

I hope some wise person can give explanation to the answer too, maybe example.

Thanks