Dear Arshad, Let me try to answer your questions that are all important ones.
Q1: Carbon sequestration is to separate carbon dioxide from the atmosphere as a method for mitigating greenhouse gases and climate change. Carbon flux is a measure of rcarbon release into the atmophere. The other two are used as frequently and I sugest you check their definitions where they used in certain context.
Q2 and 3: The descriptions are too long to put here. I suggest that you find them in the following journals:
1.International Journal of Greenhouse gas control
2.Greenhouse gases: science and technology
There are also books on the subject, which I may find for you if requested.
Most relevantly, you are invited to visit my ResearchGate page.