Hi! I think your questions about my post were replied to by other members. But as you mentioned before, if you already tried to add the IOE case to an existent account and the system indicated that the case was already added to another account, the best would be for your spouse to wait for the USCIS notice to gain access to this account that is already linked with the 751 case.
I am saying this because at this point, creating a new account under your spouse´s name will probably not work for the purpose of gaining access to this case specifically.
USCIS generally does that because, under an online account with USCIS, your spouse will be able to have access to sensitive information about his/her IOE case. Therefore, one can not just create a new account and simply add a pre-existent IOE case.
As mentioned by other members, if by any chance, you don't receive the notice by mail after waiting a reasonable time, some members reported that were able to confirm their identity through Emma and obtain the necessary information to access their online account.