Hold on.....
since you filed by mail (because you couldn't efile), the IRS should be able to sort out who otherwise filed your MIL as a dependent and give credit where credit is due.
You can't efile (because you filed via paper) and there is no need efile now you already filed via paper. Also, if the SS# has been already used by someone else, the ONLY way to file is by paper, so you did the right thing there.
you will receive a new PIN next year and will need to use that to file. Only the person with the PIN can file an electronic tax return against that Social Security Number.
I suspec the paper return has not been processed yet because the IRS is woefully behind on paper returns (think 6 months). suggest just waiting for it to be processed. It may take a little longer than that because of the issue of someone else already using the SS#?
I don't see there is anything else for you to do. Just sit and wait it out
