Accounting & Commerce

M.Com with specialisation in Banking and Finance has wider scope and opportunities. It is also aligned to your commerce & finance field. Therefore, you are advised to opt for Banking and finance.

It fully depends on your interest, as MBA has different specialisations and you can opt for your specialisation. I would suggest a name- UEM Kolkata to give it a try as this has good placement consistent record and is good in molding career to right path.

Yes, you can pursue CA after humanities but you have to focus on basics. First, you have to take CPT Entrance Exam which is held in June and December every year.
Take coaching of it and also take coaching for Basic of accounts so that it's better for your long run in CA or you can also go for CS
Other

No, most of the institutes need maths at 10+2 level. However, if you do BCA from IGNOU then you need to find a job on your own.

A Diploma is not suggested after class 12th, instead, you should opt for graduation. There are multiple options such as hotel management, travel management, medical transcription, BPO operations, etc. You should check with a reputed institute in your area.