Polytechnic

Hi,
The minimum eligibility for BBA is that you should have passed your +2 atleast with 50% and should be under 25 years of age. Various universities gives admission in BBA course on the basis of merit list. Some other college also give admission to students through the entrance tests. The entrance tests is organized at national, state or university level by various authorities. So you should be eligible for BBA based on your Diploma qualification.

You can go through the below mentioned link:
https://www.shiksha.com/engineering/colleges/distance-correspondence-b-tech-colleges-india

For that, you will have to check their eligibility criteria. For this information you can check the website or talk to them in person through helpline contacts on the website. Good luck.

12th does not come under the purview of UGC. So I guess you are safe in that regard.

Admission to the second year of Diploma is not feasible.

Eligibility criteria for the post-Higher Secondary Certificate (HSC) Diploma course is an HSC pass certificate. You are eligible to seek admission in Diploma course from a Polytechnic. Legally it is permissible to do Diploma and degree simultaneously. You can not pursue two degrees simultaneously from same or different universities. You are advised to evaluate whether you can handle simultaneous studies load of two courses and challenges in case of conflicts in time table, examinations etc.

Yes, there are some colleges which recruiters through GATE exam. But, government college not.

There's no clarity regarding this. But I'd don't think there would be any issues.

No, it's not possible. You've done a Diploma, do you've to pursue engineering after Diploma to gain full degree. You can only enter 2nd year of B.Tech CSE/IT.