@Thunderbolt hi there,
if this can help. I receive my I751 notice in July 2021. I went to Italy on vacation in December 2021 and came back beginning of January 2022. When I came back here I showed the flight attendant at Milan airport my passport, expired gc, and i751 ext letter. They knew exactly what the document was. I lended in NY airport and the officer looked at my document, gc, passport, i751 stamped my passport and let me in. He asked me how long I had stayed outside the US.
If your wife receives her I751 before traveling she will be fine. She needs her expired gc, her Thai passport and her i751 ext letter.
Hope this helped