1. Yes, you are right. As per sec 80 business loss cannot be carry forwarded if the ROI is not filed within the due date specified u/s 139(3).
2. In your case for the AY 2017-18 since the return was not filed within due date as per sec 80 this loss will not be allowed to be carry forward and set off in the coming years.
3. In the AY 2018-19, since loss return was filed within due date, loss of Rs. 28,789 will be allowed to be carry forwarded and set off against future profits.
4. All the above provision are only for the purpose of Income-tax and not for arriving at a profit under companies act. So, the loss will be set off in your books of accounts maintained for companies act and loss will be ineligible (relating to AY 2017-18) for Income tax purpose.
Please correct me if the above solution has an alternative view.