CBSE 12th

Hi,
Yes with 80%, you have a good chance. Please check the department specific entry requirement, some may require entrance test in addition to board exams. You can check this:
https://www.shiksha.com/university/manipal-academy-of-higher-education-manipal-30822.

You can apply in PGDM and you will definetely get a call for interview. More weightage will be given to your interview and personality traits and past achievements, so you have a fair chance to impress.

NATA is the entrance exam you need to appear in order to get admission in architecture colleges if you pass your 12th from science stream with atleast 50%. If you are good at maths, have good imagination skills, aptitude skills and drawing skills, you are good to go. Make sure to get good marks in NATA for admission in reputed colleges and if you want to opt for IIT, you need to appear for JEE Mains Paper-2 (Mains).

Yes, you will get a college with this percentage. There is no need to worry.

You can apply for study loan and continue your further studies in the stream in which you are interested.

It takes near about 8 to 10 months. So you should apply to it from your 12th standard. It will take three to four months after applying for it to get an offer from the college. Then arranging visa and other things take 3-4 months.