1. Will my Master degree grades be given prominence by schools or will the emphasis be on undergrad performance?
Was your undergrad in Mechanical Engineer too? The prominence would given to the story in your essays, how the Engineering eduction has helped you in your career so far and what your post MBA aspirations are.
2. What are my chances in schools like Insead singapore and NUS singapore?
I haven't looked at numbers from these schools but I would say that your profile is strong.
3. Would you suggest I re-take my GMAT.
You mention that you had strong Math grades. Looking at your GMAT quant score it might appear that you could do better if you re-take GMAT. Having said that, I am curious as to what you felt about your GMAT score. What your mock-test scores were? If you feel that you did not give your best shot on the current attempt, I would strongly suggest you to retake GMAT.
All the best