Kuh

I am assuming then that you have called the Customer Service number and tried that path to see if they have any progress on the G-884? If yes, and you have exhausted these avenues, then I suppose getting a duplicate is the only option?

I think there is a form you can fill to request return of your original documents if filed for an immigration benefit. You might want to try that path first. I don't know if this will help, but here is a link. It mentions G-884. 
https://www.uscis.gov/forms/file-my-application-online-e-filing/how-do-i-know-if-i-need-original-documents

hi
 
do you mean you sent the original certificate? because you weren't supposed to, never send the original certificate for an immigration petition, you only needed a copy of your certificate to prove that you are a USC
 
now you will need to file for a replacement, since immigration won't return it to you. it's in your mom's case file