The 2023 carrier guide, page 32, states a person with a 10 year expired card can board. Whether a particular airline will honor that is another story. I agree she would not been applying for an immigrant visa. If they denied her a boarding foil, she is not in a good situation.
US immigration requires a great deal of knowledge, planning, time, patience, and a significant financial investment. You are on the right track. Good luck.
Another Option: Marry now, and start the DCF (Direct Consular Filing) spousal visa process a few months prior to desired move (with exceptional circumstances such as a short notice job offer for US citizen).
Another option: Marry now, and start the spousal visa process. No adjustment of status needed. In about 2 years, he can enter the US as a Green card holder.
In my opinion, she should have just gotten on a plane and returned to the US. She is authorized to board a plane with an expired 10 year card. The worst case would have been to argue her case in immigration court.....or she could have been admitted with NO court appearance. Is her appointment in Lagos? That is the WORST consulate in the world to argue a case, imo. They are tough, and they could easily deny her case. SB-1s are not given easily anywhere. Then what is she going to do?
Good luck. Keep us informed.
I agree with @OldUser and @Edward and Jaycel. You must have filed for most recent tax year, if required (and the two previous, if required) to qualify as a sponsor.
DFW is pretty straight forward for foreign arrivals. Customs, immigration, and baggage area are all in a secure, restricted area. There is only one exit point into the public area.
It's a problem, imo. You need to make the decision easy for USCIS. They clearly don't like what you sent as supporting documentation for the I-864. The path of least resistance, imo, is to find a well-qualified joint sponsor to submit an I-864 and solid supporting evidence. You won't get another chance. I agree with @OldUser.
At this point, I would certainly get my Senator's office involved. They should be able to, at least, get a response from the consulate, as to what, if anything, can be done.