CMA
13117 Points
Joined May 2009
1) Yes, it is mandatory to mention 4-digit HSN in case of B2B supplies w.e.f. 1st April 2021.
2) I think HSN can be added by amending same in the next GSTR1 (though not sure)
3) General penalty of 50k u/s 125 (25k + 25k) would be required to be paid in case of receipt of notice from the department. I think chances of getting such a notice is not very certain (unless repetitive) because there is no loss of revenue to the dept.