MAT

Yes, Diploma students can appear for these exams.

Chandigarh University has all the approval AIU association of Indian universities and Wes, UGC, AICTE, NAAC, etc. So your degree will be globally acceptable Chandigarh University Institute of Distance & Online Learning is recognized by the University Grants Commission (UGC), a statutory body of the Government of India established for the coordination, determination, and maintenance of standards of university education in India.

The result will be declared after 2 or 3 months.

Yes you do have a chance to get into Christ University but first check the cut off list and find your name there then only you will get admission.

https://www.shiksha.com/exams/ignou-openmat-exam-syllabus
Check here.

Amity University, GD Goenka, and Galgotias University would be a good options for you.

Yes the generally fee will be low if you get through TANCET.

Yes, if you have given the exam recently, then it is definitely valid.