Sorry....now that your wife's SSN has been put into the system on a tax return, it will stay there for months. When someone tries to e-file and claim her as a dependent, their e-file will be rejected with an explanation that says the SSN has already been used. So, before that happens, sit down and talk with your wife and figure out the best way to tell her parents.
As for amending... if claiming her as a dependent for 2019 is still something they insist on, you can amend to MFS as long as it is done before the filing deadline of April 15, 2020. They do not have to wait for that; but they cannot e-file. They would have to print their return claiming her and then sign it and mail it in.
A tax return cannot be amended to change from a joint return to married filing separately after the filing deadline of April 15. You say you already sent in a 1040X---are you sure that is what you did? Did you use a 2019 Form 1040X form and print, sign and mail it? Where did you get it? An amended return cannot be e-filed. It has to be printed, signed and mailed. The 1040X for 2019 should become available later in February.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
