Green

There's no erasing the mistake already done as the interview is over and the 221g already sent.  The CO already knows that the charge was not disclosed in a timely manner.  Can't remedy that now with a new DS-260.  The CO wants the document requested in the 221g.  Submit it ASAP.
 

I really don't know if writing a statement at this point would have any value.  It's possible that the consul officer may take it as a desperate effort on the visa applicant's part now that she has been caught in a lie.  But I suspect that the decision on that matter will be based on the applicant's reaction during the interview.  COs are trained to read body language, so it's also possible that the CO will think that it really was just an honest mistake if the applicant seemed genuinely surprised when the omission was brought up at the interview.
 
Anyway, what's most important now is to submit the document requested in the 221g ASAP.
 

It's too late to change the DS-260 now as the interview is done.  The consul officer will make a judgement on whether the omission was willful and material misrepresentation.  Submit the requested document and wait for the CO's decision.
 

It should be one I-864.  Family size would be 3 if it's just you, your fiance and stepchild in the household.

Hopes aren't very high after the last one, but there's this tomorrow, being run by the Venezuelan virtual embassy in Bogota.  Encourage anyone to join and get information, put pressure on them to open - although not entirely sure of what power the Venezuelan virtual embassy has, but they are operating out of the Bogota embassy where Venezuelans go for interviews.
@David G
 

I called the Embassy in Nov.....they told me if they dont have open slots....they can extend it.