Distance/Correspondence

Yes, you can do B.Tech through distance. It will be very beneficial for you to do B.Tech after completing Diploma.

You need minimum intermediate qualification with 45% or more to pursue BCA.

Whether it's Correspondence or part time MBA, it takes duration of 24 months to complete the course. However many European countries offer one year MBA programs like the UK, Switzerland and France.

Hello Sihaib,
It's indeed quite good to hear from you that you are interested in doing graduation after completion of the diploma. Yes, there are plenty of institutions that offer this four year course. I am sure you will be greatly benefited by this. I am attaching a link that will give you all the required information.
https://www.shiksha.com/b-tech/colleges/distance-correspondence-b-tech-colleges-india

You can take admission directly in 2nd year of Diploma in mechanical engineering on the basis of ITI. Almost all the Diploma colleges provide lateral entry in 2nd year.

IGNOU is one of the best amongst distance learning institutes and admissions are open, you can apply online.