deceased did not take an RMD for this year - IRA is already dispersed

hi - The situation is that the deceased did not take an RMD for this year. All of the beneficiaries (20 people) have their portion in their inherited IRA's at this point.
 
- How should the RMD be taken?
- Who's responsible for making sure the RMD is done?
- what are the ramifications tax-wise to the deceased taxes if it's not done?
- If one person cashes in their inherited IRA and it's enough to cover the full RMD, how can this be documented for tax purposes?
 
Sorry if I'm making this too complicated! thx