“Once USCIS and NVC approve the case, and the case is mailed out to the beneficiary’s local embassy for an interview, should the interviewer still look at adjusted gross income or gross income?”
For the fiancé visa, absolutely. For the K-1 visa, which is technically a non-immigrant visa, the U.S. Embassy officials their staff who evaluate your finances. So that will be the first and last time your finances are looked at for a fiancé visa. For other immigrant visas, like a spousal visa, the NVC is going to look at it. So you’ll never get to the embassy interview stage until your finances have been approved and then they will typically not review them again at the Embassy. If you’re on a CR1, IR2, IR5, or one of those family visas where you’ve submitted your affidavit of support to the National Visa Center and now you’ve cleared the National Visa Center and they send it to the Embassy, you’re usually, at that point, past your financial obligations.
If you take too long, utilizing extensions, from the NVC stage until you interview, they might want to look again and make sure you’re still employed. It’s possible, but it would be unusual and it’s not normally the case.
The original question also touched on whether or not they look at gross income, and adjusted gross income. They look at total income, which is found on line 9 of the 1040 form you file with the IRS for your taxes. In most cases, if you’re just a regular W2 employee, you’re going to be looking at gross income. If you’re self-employed, an independent contractor, then they’re also going to be looking at total income.