a!writeToMultipleDataStoreEntities at interface for primary key

I think it would be difficult as the key (idPk_int or id_int or idPK, etc.) of the entities will be different and hence you will have to include all of them so you do not miss out on any. What are you getting in 'local!emplEntity' after the smart services runs successfully? Can you attach a screenshot after expanding this variable? 

It should not be an issue if primary key names are same for all the entities. a!writeToMultipleDataStoreEntities() returns list of entityData type, so you can index or property the keys like you do with any dictionary or map.

Here is a sample code: 

a!save(
  local!primaryKeys,
  a!forEach(
    fv!storedValues,
    property(
      property(fv!item, "data", null),
      "idPk",
      null
    )
  )
)

The documentation explains how to extract data from the returned values.

https://docs.appian.com/suite/help/23.2/Write_to_Multiple_Data_Store_Entities_Smart_Service.html#node-outputs

You are using writeToMultipleDataStoreEntities. so to get the value of the inserted/updated data, you have to use fv!storedValues[1] instead of fv!storedValues. [1] is the entity for which you want to get. it is counted as sequentially. For example: in writeToMultipleDataStoreEntities you update 3 tables and you want to get the data for 3rd table so for that use fv!storedValues[3]