BCA

For B.Com it entirely depends upon the specialisation which you are pursuing. For example if you are pursuing B.Com in Finance and Accounts, its quite difficult whereas B.Com in marketing is comparatively easier. It also depends upon the university. Calcutta university is among on the most difficult university to pursue your B.Com.

Yes, you are eligible for that.

BCA is far better than the Polytechnic degree, because after completing Polytechnic, you will just get a Diploma degree, but BCA is a Bachelor's degree and after completing it, you will eligible to fill any government forms of graduation level.

BCA Course: Eligibility
Candidates who wish to pursue BCA course must fulfil the below mentioned eligibility criteria:
Pass in 10+2 examination with any stream from a recognized board of education.
English as a subject of study.
A minimum aggregate score of 45% at 10+2 level.
Some colleges prefer students from science stream or those who have studied mathematics and computer science at 10+2 level.

Yes, you can take the BCA entrance exam of National PG College (NPGC). For more details, check the link below:
http://www.npgc.in/Admission-Eligibility.aspx

Admission is through counselling based on the marks obtained in 10+2. Interested candidates can apply online with the required information. The minimum eligibility to apply for Bachelor of Computer Applications in Kristu Jayanti College (KJC) is that candidate should have a minimum pass mark of 45% in class 12 with maths as a mandatory subject.

Higher study options in Computer Application after BCA:
There are many options for BCA graduates in higher studies. Some of the popular higher study options are mentioned below:
Master of Computer Application (MCA)
Masters in Computer Management
Masters Degree in Information Management (MIM)
Masters in Business Administration (MBA)
Master of Science in Information Technology M.Sc. IT)
G-CAT Master
Postgraduate Diploma in Computer Applications
Post Graduate Programme in Corporate Studies (PGPCS)
Postgraduate Diploma in Management (PGDM)
programme in Information Security Management
Regards.