M.Com

Yes, you can definitely opt for M.Com in your Master's degree. Kindly read about the comparison here:
http://management.ind.in/forum/mysore-university-vs-bangalore-university-171217.html

No, you only need to appear for a single exam which you failed to clear earlier.

Nothing. You do not get a job by adding a degree. Every job has a specific requirement if you are applying for a job for an accountant then MBA is overqualified and it means that your MBA has no value (it also depends from which institute you have got your MBA and if an MBA cannot fetch a campus placement for you, in the market, it has no value and the job you get will solely depend on your own ability, not on the basis of MBA). If you are applying for a job such as sales, marketing where MBA is prerequisite your M.Com has no value as it is a non-professional degree.

Hi Puja,
It's not available till now.
I hope you will get it soon in the website.
All the best.

Your choice should depend upon the nature of the job you would like to do or the kind of career you would want to make. However, I would always prefer M.Com first because it would enhance your knowledge of commerce subject that you have chosen and also because MBA can be done even later if you find the need for it. But if you are not interested in making a career related directly to commerce then opt for MBA.