Can I join engineering after diploma in Mechanical engineering? What are the other course options available?

Karthik, after NFT, you can not even get admission in diploma as that too is technical. I would suggest to go for BCA+MCA route if you have inclination towards computers and software. Second route can be B.Sc.+M.Sc. Third can be BCA+MBA

Just complete diploma and join AMIE (I) as this will save your time, money and you can join a job simultaneously. This will fill that gap which you lost during degree. AMIE (I) is professional qualification, recognized directly by government and is very reputed. AMIE (I) is a qualification by examin

Technically speaking you can join engg again by the usual process, but before that you need to deeply think and analyze if that is the only option for you. In my opinion you must now think differently. The hurdles in your engg are indicatve of something, which must understand and so change your path