Deadline cannot be extended... you should be submitting a complete application else it will be rejected..
If you still need more time I suggest decline the ITA which will bring you back to the pool so you can get a new ITA in the next draw, if you are over and above the cut off
I’m not sure about the affidavit if it will work or not..
Marriage certificate Purpose
To validate marital status claims
Document requirements
This document is requested only when marital status is listed as married.
The marriage must be valid under the law of the country where it took place and under Canadian law.
A marriage performed in an embassy or consulate must comply with the law of the host country where it took place, not the country of nationality of the embassy or consulate.
If the applicant married in Canada, a copy of the marriage certificate issued by the province or territory where the marriage took place must be provided.
If the applicant married outside Canada, a copy of a legal certificate issued by the country, the territory or the region where the marriage took place must be provided.