B.Com

Yes, As advertising is one of the modes of Marketing
you can pursue your career in marketing by doing MBA or Diploma in advertising.

It depends upon you. If you don't have any pressure of earning you can go for higher studies otherwise you can go for a job. Along with the job you can learn through various correspondence courses.

You can go for both. While doing M.Com on regular basis. You can do a Diploma on distance basis. You will get do degree/certificate at same time. Government validates getting 2 degrees at same time one as regular one as distance.

You should either go for MBA. Or various institutions provide PG Diploma in taxation you can learn from there.

Yes, you can. There's no restriction on doing so.

CA somehow make your B.Com degree valueless. But doing B.Com will help you in covering more fields which will help you in the future after becoming CA.

It depends upon you if you want jobs you can go for jobs. If you want to do further study you can go for M.Com MBA or professional course like CA CMA CS and many more.

No, you need to bachelor's in psychology to get admission to a master's. You can still do a Diploma in psychology.

It depends upon how many attempts you take to clear your exam. Normally it takes 2-3 years for students to clear it in 1-2 attempts. The more attempts the more it will take time.

MBA or Diploma in marketing. Now many edu techs also provide tailored advertising courses. You can do certificate courses from them.