CS Examination

The scope is pretty good. If someone has done CS and law, just apply to any big law firm or in any company with this qualification and you can get a handsome package.

It is always beneficial to do both because you get two individual yet connected degrees. If you want to pursue CS it can be always combined with another Bachelor's degree. If you can complete both of them side by side without compromising on performance in any one you will be able to set up a strong career for yourself.

If you want do an integrated course again, then go for one of the nationals by taking CLAT or AILET.
And for a 3-year LLB thing, you can join Faculty of Law, DU, BHU, AMU, JMI and other state universities.
Moreover, you can do CS simultaneously.

Look don't try to get involved in more than one thing simultaneously.
That would affect your career too.
Try to focus on one thing only and do your best in that particular area.
Good luck

No idea about CS, so can't comment. But for MBA, yes, the academic scores especially the graduation marks do matter as a lot of top B-Schools give significant weightage to them. During the interviews, one can be questioned about his graduation marks (if low). Also, once getting admitted into a particular B-School, there are companies who consider the graduation marks seriously while recruiting for summer and/or final placements.

The main difference between CA CS studies is that in CS, you are going to learn more about company law and in CA, you are going to learn more about accounting and taxation.

You may refer to the following link to get an idea about salaries of company secretary in India:
http://www.payscale.com/research/IN/Job=Corporate_Secretary/Salary

MBA after B.Com is a good option because it has good job opportunities provided you do MBA from a top colleges. Otherwise B.Com+CS is also a good option.