1)when is the right time for closing fast….is it 13 min before fajr or at fajr?2)can the fast be opened right after praying Maghrib salah with water…..then continue with Ishaa namaz?

You should use precaution and stop eating a few minutes before fajr.

Customarily you end the fast at the adhan of Maghrib then make salat al maghib and isha

Rule 1559 from Ayatollah Sayed Sistani says it is better to wait a little bit after Maghrib has set in as a means of precaution.