Exam

Yes, you are eligible.
Thank you.

You should retake the exam next year. With this score you have very low chances of getting good colleges.

It would be difficult considering your score. The cut-off for Mumbai campus is above 75 percentile. You may have a slight chance in Bengaluru campus.

Most of the good colleges have the fee structure in the range of 10-15 lakh rupees. So, it is difficult to get into a good college with your budget.
For more details, kindly refer to the link mentioned below:
https://www.shiksha.com/colleges-accepting-cat-scores-range-70-to-80-percentile-article-1195

You have very low chances. The CMAT cut-off for MBA in GIM will be around 96-97 percentile.

Check srmuniv.ac.in. Its syllabus is almost similar to IIT-JEE Mains syllabus. Well, preparing for IIT-JEE will make it easy for you to clear SRMJEEE.

IIM A do not admit students from CMAT. One has to clear CAT or GMAT to get admission in IIM, Ahmedabad.

Start revising the basic maths concepts studied in class 9th and 10th. CAT generally asks questions on profit-loss, statistics, trigonometry, ratio and proportion, basic algebra. Verbal needs practice and same is the case with DI-LR. You can also join a good coaching institute if possible by April/M

If you are targeting this year's CAT then you must start the coaching by April/May. 6-7 months of preparation with full dedication and commitment is good enough. Do take weekly tests to evaluate your preparations.
Best of luck.