M.Com

It is an excellent choice. You must do MBA in marketing. There are many job opportunities for you in the marketing field.

Yes, there is no problem but 2-degree courses done together at the same time have no value.

Yes of course, you are eligible for this course.

Tamil Nadu Open University, Chennai
Master of Commerce - M.Com
PG Degree | Distance/Correspondence | Duration - 2 years
Total Fees: INR 3,000
Download their brochure from the link mentioned below for further details regarding admissions, course eligibility, etc.
https://www.shiksha.com/accounting-commerce/course/master-of-commerce-m-com-tamil-nadu-open-university-anna-salai-chennai-113363

Kerala State Eligibility Test (SET) is organised by the Government of Kerala in order to ensure that teaching standards are maintained for courses taught at Higher Secondary level in the state of Kerala. Thus, in order to be appointed as Higher Secondary School Teachers as well as Non-Vocational Teachers in VHSE, candidates need to clear Kerala SET on priority.
Please refer to the link provided below for eligibility for SET exam.
https://www.shiksha.com/teaching-education/exams/Kerala-set

Don't go for CS and CA. Passing percentage ratio for CA and CS is quite low. Even I was pursuing CS, but could't clear CS executive. So, I took decision to do MBA in finance, which proved a good decision for my career and got job at Deloitte. So, take your decision accordingly. Doing MBA will give you more job opportunities as per my knowledge.