a. Considering the academic stats, does it makes sense to take extension classes?

Not really. Your GPA is not bad and your GMAT is great. I feel taking classes is too passive. You ned to find other things to work on.

2. I am confused with the reapplication essay. How do you demonstrate that I have significantly done better from the last year I applied in 6-7 months (I applied to many schools in Round 2 and planning to apply in Round 1 this year). In what areas can I demonstrate that I am a better candidate this time around?

This is hard, I admit. Basically you have to PROVE improvement in as many areas as possible. It can be though a promotion, a raise, new responsibilities, newer bigger projects or clients, new achievement, higher test scores, more leadership, more volunteerwork. Anything you have in your profile that can be improved, in fact. But most effective are things with RESULTS