YES Education

A commerce student can also join army. The candidate must pass stdxii. But directly after XII he can't join army He must appear for NDA Entrance Exam. NDA the National Defence Academy, entrance exam is conducted by UPSC twice every year as NDA 1 and NDA 2 for entry to Indian Army.

As of now being a commerce student, you cannot get admission in B tech courses, however, in order to pursue engineering, all you need to do is to take up Diploma in Engineering. There are many universities which offer Diploma in engineering, after which you can get into lateral entry admission in B.Tech courses.

Yes, you can do after B.Com, as you already have Bachelors degree you can either opt for Diploma in Interior Designing ( 1 Year programme) or M.Sc. In Interior Design considering the fact that you already have Bachelors degree.

Yes and you need to pay a caution bond money of 11500 which is refundable at the end of course.

There are many UG exams for admission in BHMCT course state wise .
Few exams are mentioned below:
NCHM JEE
Aktu Exam for UTTAR PRADESH Colleges
And many universities have their own entrance tests also.