What can happen if we are filing our I485 with the USCIS but are worried it won't be returned in time for her to travel home?

To re-enter the U.S., a foreign national either must have Advance Parole (a "travel document") obtained through her marriage-based adjustment of status application process, or a valid nonimmigrant visa. Once an I-485 application has been filed, an applicant will be unable to re-enter with a B1/B2 visitor's visa, even if the date on that visa has not yet expired - this is because entry with a B1/B2 visa requires "nonimmigrant intent" and the filing of an I-485 application demonstrates "immigrant intent." Even if your wife were to delay the adjustment of status application process, she almost certainly would be unable to re-enter the U.S. with her B1/B2 visa because (1) she is now married to a U.S. citizen, and (2) she indeed does have immigrant intent. It would be wise for you and your wife to work with an immigration attorney in the application process to assure that the applications are properly prepared and supported with a full set of supporting documents - errors or insufficient supporting documentation can cause significant USCIS processing delays and even more harsh consequences.

She definitely can be denied entry and returned to the U. K. Or last port of departure. She must have an advanced parole card/document. Otherwise, she may miss her interview and the adjustment will be denied. I strongly recommend an appointment with a competent and experienced immigration attorney before there are more complications. There are other options, but these may not meet your expectations. Good luck. The above is general information, not legal advice, and does not create an attorney client relationship.