c.   File separate extension requests for each spouse.  Filing a joint 4868 isn't a commitment to filing a joint return, but it doesn't offer any more protection and might avoid questions later.  Follow the Form 4868 instructions, " If you don’t have an ITIN, enter “ITIN TO BE REQUESTED” wherever an SSN is requested."

Did the US taxpayer already receive a $1,200 EIP based on 2018 filing status and income?  Filing a joint return with a spouse using an ITIN could eliminate it on a 2020 return.  This might also be a problem with future EIP "second round" legislation.  CIS may like to see joint returns for purposes of green card and citizenship applications, but lack of that evidence this year can easily be explained (by you, if needed).