BCA

No. To get admission, there is no entrance exam, but marks criteria will be 55% in your Bachelor degree.

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.

You can get direct admission, but for the good college, where some placement also available, you have to give an entrance exam.

Top colleges in India offering a course in BCA are mentioned in the link provided below. This ranking is based on college rankings published by India Today. Link:
https://www.shiksha.com/it-software/ranking/top-bca-colleges-in-india/96-2-0-0-0.

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