Diploma

One degree and another Diploma is allowed or legal.

If you are interested in doing so, you can go for it.

NIFT is one of the best college.

HI Rajbir, University based dental programs are either bachelor or master degrees. Diplomas in the form of associate degrees which are entry level programs are offered by community colleges. For BS programs in Dental Hygiene you may check Northern Arizona University, University of Maryland, Ferris State University, University of Michigan, Oregon Institute of Technology, University of Pittsburgh to name a few.

It depends on your previous qualification, you must check if you are eligible and which is better for your profile. For long term prospects a Bachelor's degree is preferred.

Yes, it is a good decision. Marketing is based on the field work during initial stages like sales.