Mnthly return gstr7

There is no such official notification. 

However it is expected that the return can be filed upto 30.06.2020

Yes ...As per Notification 35/2020 CT , Where , any time limit for completion or Compliances date is falling from 20/03/2020 to 29/06/2020 shall be Extended to 30/06/2020.

So the last date to file gstr7 for march is 30/06/2020

Yes the due date for filing FORM GSTR-7 has been extended to "30.06.2020."

As per Notification No. 35/2020 - Central Tax, dated 03.04.2020, the due date of all the compliances falling between 20.03.2020 to 29.06.2020 stands extended to "30.06.2020." Hence GSTR-5, GSTR-6, GSTR-7, GSTR-8 extended due date is 30.06.2020.