JulesInCNY, the AP is required to start taking RMDs in the year the Participant hit his required beginning date (so the RBD for the AP is based on the participant's age, employment status, and 5% ownership status, not the AP's info).
As far as your 4/1 question, the participant hit his RBD in the year before the QDRO split occurred, so the AP would have to take her RMDs by 12/31 of each year (i.e., no 4/1 delay since the participant's RBD occurred in an earlier year).
So, if AP received a portion of his account balance in 2017, AP should have received RMDs by 12/31/2018 and 12/31/2019 already.
If you know the participant is still alive, the AP's RMD is determined by dividing the AP's prior year separate account balance by the Participant's uniform lifetime table factor based on his age in the year of distribution (rather than the AP's uniform lifetime table factor using her age in the year of distribution). If you know the participant died after RBD, then the AP's RMD would be determined as if she was a surviving beneficiary of the Participant - that is, by dividing her prior year separate account balance by longer of the remaining single life expectancy of the participant (determined based on age in year of death and reduced by 1 each year since the year of death) or the single life expectancy of the AP (determined based on AP's age in current year). Hope this helps.