B.Com

After B.Com, you are not allowed to pursue any courses related to science, unless you've studied science stream at PU or at class 12th.

There are many courses available in the market, which you can choose. But, the main point is your interest and capability. So, I would suggest you to go to any specific institute and talk to the counsellor about the courses available there. Through this, you will get an idea about the same, and then you can choose accordingly.

After completing B.Com, there is no science course available.

Yes, you should go for it. You should try to get admission in a better college.

You can start practicing problems on verbal, quant and DI/LR. You should make a habit of solving problems from each section daily. In quant and DI/LR take time to solve each problem thoroughly and keep note of concepts and formulas used.
For verbal and RC practice is required and variety of questions are needed. If you want, you can join a coaching class as they will assist you in your preparation in a planned manner and weekly tests will help you to assess your performance.
Good luck.

You have already done DIET and it makes you eligible for school teaching. Now, you can go for MBA or M.Com for other career options.

After B.Com, you can get a good job in the accountancy field. You can pursue CMA course along with B.Com. You don't need to pursue MBA afterwards.

If you have a keen interest in numbers, opt for accounts, and if you have a keen interest in theory courses, opt for a management course.