Degree

All colleges grant you admission either on merit base that is, class 12th marks or on entrance based.
Try your luck, if 10 marks works, but I don't really think so.

I am not really sure that you can pursue two degrees at a time. Please take help of some elder which could help you better.

Hi Pramod,
Yes, B.Des is, in fact a Bachelor's degree. FDDI is affiliated to Mewar University, and you will get the degree from Mewar University itself.

It is good enough. But, generally doing MCA after BCA is often preferred.

There are Entrance exam for some of the universities. However, he/she will be considered for admission if he/she fulfils the other requirements of admission. I think, yes you are eligible to do M.Sc Maths after B.Tech (ECE), but mostly it depends on the university, where you are going to pursue Master's degree.

Depends, when you will clear your backlog. Suppose you clear your 6th semester this summer and having back log of 3rd semester, and you clear your 3rd semester this summer, then your passing year will be summer 2018. If you clear your backlog in the months of winter, then also it will be 2018 pass out, but the degree will show winter 2018 pass out, but this shows you lag behind others those who have completed their degree in 3 years, and you took 3 yrs 6 months for completing. This will put bad impression on your degree. Try to clear you backlog with 6th semester. Best of Luck.

B.Sc Computer Science is not equivalent to B.Tech because, in B.Tech, you study more subjects than B.Sc. But you can still get a job on what you have studied.

If you want to opt for B.Tech then do study till class 12th. It will prepare you for the coming years of your B.Tech. See if you want to end your school in class 10th and start pursuing higher studies, that is fine but it is your choice. Since you are saying you want to do B.Tech then I would suggest you complete your 12th and then directly do B.Tech. You can opt for Diploma after 10th and then do B.Tech through lateral entry.

It completely depends on the university, generally the universities don't allow to continue after such a long gap. Although, we can't say what the university decide.

Go for RULET or crack any three year law course entrance examination. Do up vote and follow, if my answer helped you in any manner.