RC cancelled

Since there is no tax liability, interest won't be payable but you will have to pay late fees @ Rs. 20/day for GSTR-3B. At present, the portal is not calculating late fees for GSTR-1, but that will start after 10.01.2020.

After payment of dues and filing of returns, you can apply for revocation of registration.

Yes.

Late fees is calculated automatically by the portal.

The late fees mentioned in a particular month GSTR-3B includes the figures of the previous month return filed late.

Suppose you file Feb month return after due date. The late fees will not be included in the Feb GSTR-3B but it will be mentioned in the Mar return.

Late fee for the month includes previous month’s late fee charged due to delay in filing of return. The calculation is [Date of Filing – (Date of Submission or Due date of Filing, whichever is later)] * [25/day (in case of any liability) or 10/day (in case of nil liability)] * per Act (CGST/SGST)