Exam

There is no MBBS course provided by National Defence Academy or NDA. If you wish to pursue MBBS from an Army Institute and serve in the army, you can fill the form for AFMC Pune or Armed Forces Medical College, Pune. Admission in AFMC, Pune is done on the baisis of your NEET scores only

Big yes, of course. It does not matter that where are you completation of your graduation, either open or Regular university, the valuation of the degree is always same as per UGC guidelines. So, go for it.

I don't know about improvement exam but for JEE mains:-
Unlike India, Germany has 13 years of school education and having qualified JEE Mains means, you are qualified to take admission in German universites but if you are like me who is not from IB/Cambridge and didn't qualify JEE then to study in Germany either you have to do studienkolleg in germany or complete 1year of bachelors 

Yes, either XAT or either CAT, MAT or any other National level entrance exam score required to take admissions in St Xavier's University in MBA program. 

After that, you have to qualify the interview round and group discussion.

After that, you will be shortlisted on the basis of your score and your performance in interview round and group discussion.

Yes MAT is required for admission at Dr. C. V. Raman University and it depends upon which course you are selecting for your admission. MAT is required for the M.Tech and MBA student and other exams for these courses are CAT and GATE.  
For other courses, there are different exams which are acceptable for the admission like for B.E. JEE Mains and PET and for Polytechnic and tech student must have clear CRVU CET. 

Yes, St Xavier University accepts MAT entrance Exam score for MBA program.

•Besides this exam, this university also accepts admission through CAT, XAT, and any other National level entrance exam paper.

After that you have to pass through written test, Interview round and group discussion.

After all, you are eligible to take admission.

Internal exams are just tests that are conducted so you can score good marks at the end of the semester. Nothing happens if you fail, you just have to study well so that you can score good marks in semester exams and have a good aggregate of marks