Commerce

Yes, Mathematics is a necessary subject as CAT has one section dedicated to Mathematics. Also half the courses that are being taught in IIMs are maths based. However being a commerce graduate you are not at all in a disadvantaged position. There are a lot of commerce graduates studying in IIMA. They

A degree from a private university (holding ugc/aicte accredition) in regular mode has same value as a degree awarded by government university. But, a regular degree has no comparision with part time/distance mode. Regular is much much more acceptable in industry. All the best

there can be many like bba , b.com ,etc. after that u may pursue MBA

The job opportunities are not very exciting after bcom
You could get junior accountant job with low pay of 10000pm
But you need to know advanced excel and tally so you could learn that
other than that mostly sales jobs are offered
So learning further would be good
professional courses like company secret

Doesn't matter which line you choose. Just you need to complete your graduation. Arts might be helpful to you.

You can go for post graduation courses like mba CFA
Do a job and then try certificate courses like CFP CCRa
you could try internship to know if you wish to do a job or study further
you could start applying and intern somewhere until your results come out and then decide