My question for you is what happened at your spouse’s Adjustment of Status interview? Did USCIS approve her case? As I see it, three possibilities exist:
1. USCIS approved her case, but you have not gotten the card in the mail yet
2. USCIS did not approve her case and has continued the matter.
3. USCIS did not approve her case and has continued the matter. She travels with the I-512L and USCIS approves her case while she is outside the country.
Option 1: If USCIS approved her application and she has yet to receive her green card, it would not be appropriate for her to travel with the I-512L. The I-512L is for green card applicants. If USCIS has approved your spouse’s case, she is a green card holder. You should have just received in the mail Form I-797 (the I-485 approval notice). Please note that your spouse cannot travel outside the country with the Form I-797 (I-485 approval notice). To travel, she must receive the green card and have a current, unexpired passport. She must travel with both of these documents. If she has not yet received the green card and must travel due to an emergency, she must make an INFOPASS appointment at her local USCIS office to receive a temporary green card stamp in her passport. The day of the appointment, she should plan to take the original Form I-797 (I-485 approval), a current, unexpired passport, and documentation proving the emergency. If you decide to make an INFOPASS appointment, you must make the appointment online at the following web address: http://infopass.uscis.gov/.
Option #2: If USCIS has not yet approved her application, she is still a green card applicant. She should use the I-512L and a current passport to travel while her case is pending. U.S. Customs and Border Patrol (“CBP) would admit her with this document. It would not come down to an officer having a bad day. The I-512L is valid and legal as long as it has not expired.
Option #3: This involves various ‘what if’ scenarios. I am sure more scenarios exist than come to my mind. If you are here while your spouse is abroad and you receive her green card, I cannot think of any prohibition against you express mailing the card to her and having her re-enter with the card. Otherwise, she would re-enter with her I-512L. CBP may send her temporarily to secondary inspections while they verify the information. They should work with her on this to process her as quickly as possible.
I hope this helps.