Architecture

Architecture Is not a crash course. You have to restart from diploma architecture or based on your 12th exam and NATA score which is a separate exam offered by COA India. You have to pass and get NATA ranking based on that. You can join architecture colleges in India.

Yes, you can as NIOS is a recognized board in India.

Yes, you can take admission in architecture after completing your class 12th.

There are a lot of question banks available for NATA exam preparation, any of those books can help you but questions vary every year and will not be the same. The question banks or any other books will only guide on what lines the questions in NATA can look like.

IIA is Indian Institute of Architects where only Architects can be registered.

If you plan to work abroad, doing Masters abroad is a better choice, but if you plan to work in India doing Masters in India is the best, for a simple reason that planning norms vary from abroad to Indian conditions.
I would say it varies from one individual to other, some prefer to do Masters as soon as they finish B.Arch for a simple reason that the connectivity to studies is not lost in the due process. Where as others feel with work experience Masters would be easier a task.

An Architects career oppurtunities vary from being an Entrepreneur to the following:
Architects (Junior/ Senior)
Furniture design
Photography
Construction& Community based volunteering
Infrastructure development
Graphic design
Set Design
Jewelry design
Advance research in Architecture
Faculty @ School of Architecture
History, Restoration and Preservation
Interior design
Product design
Landscape Design
Project/ Construction Management

Architecture as a course does not have any placements. You need to send your portfolio to your desired firm to be selected. If you are asking about the best colleges - CEPT Ahmedabad, SPA Delhi, IIT Kharagpur, RV College Bangalore, BGS School of Architecture and Planning Bangalore are the best.