There is a US embassy in Mozambique, but it says "pause on non-immigrant visas", so NVC may indeed have sent the case to South Africa for the interview.
https://mz.usembassy.gov/visas/
It's not clear why that would be, nor whether K-1 is included in the pause. Mozambique is not on the latest group of countries where non-essential travel is being denied.
You said both petitioner and beneficiary are Venezuelan. Venezula IS on the ban list, and if the US government is not accepting official documents from Venezula, then your friends may have a problem. I'd chase down whether their birth certificates, background checks (will be needed from Venezuela and all countries of employment including Mozambique), and other vital records are even going to be accepted before arranging beneficiary's travel to Johannesburg.
Having an interpreter definitely seems like a good idea because the document check has high potential to get very complicated. This isn't a DIY situation. There is probably no hope of success considering the ban looks at nationality, not residence or where the interview takes place.
It's also probably worth asking for a review of the full document list (don't post copies of the documents, but e.g. title, issuing agency, and date of issue for each one is important information).
Do they have dates set for document review, medical exam, embassy interview? How fast is the document review approaching?