Can I do B.Com directly from the 2nd year after my Diploma in mechanical?

No. You can get admission in first year though. You can only get lateral entry in BE or B.Tech. Your Diploma will be considered equivalent to 12th for B.Com.

To seek admission into B.Com, you will first have to complete your 12th & then take admission into first year B.Com and then go ahead. So, in case, you are pursuing Diploma post 12th standard then you have to first take admission in FY B.Com and then proceed further.

No, you have to start from the 1st year itself.