InitialExpression is what you are looking for:
https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cl…

In my opinion the best approach is to create a BO for each database connection and invoke them from a BPL, getting the results and doing whatever you want. 

UserTable and DataTable are related? I'm guessing that if Condition1 is the relation between the tables you would try this query:

SELECTCOUNT(CASEWHEN data.a = "Condition1"then1ELSENULLEND) as"ValueA", 
    COUNT(CASEWHEN data.b = "Condition2"then1ELSENULLEND) as"ValueB", 
    COUNT(CASEWHEN data.c = "Condition3"then1ELSENULLEND) as"ValueC", 
user.id
 FROM UserTable userleftjoin DataTable dataon user.id = data.user

I wrote an article about the creation of a REST service that maybe can help you.

https://community.intersystems.com/post/creating-rest-service-iris

You can check the ClassMethod TestPost where a data is received from a post call and is sent finally to a business operation where is transformed into a DynamicObject and parsed to another class. This is the business operation:

Class WSTEST.BO.PersonSaveBO Extends EnsLib.REST.Operation
{

Parameter INVOCATION = "Queue";

Method savePerson(pRequest As WSTEST.Object.PersonSaveRequest, Output pResponse As WSTEST.Object.PersonSaveResponse) As%Status
{
	try {
      set person = ##class("WSTEST.Object.Person").%New()
      #dim request as%DynamicObject = {}.%FromJSON(pRequest.JSON)
      set person.PersonId = request.PersonId
      set person.Name = request.Name
      set person.LastName = request.LastName
      set person.Sex = request.Sex
      set person.Dob = request.Dob

      set tSC = person.%Save()
      set pResponse = ##class("WSTEST.Object.PersonSaveResponse").%New()
      set pResponse.PersonId = person.PersonId
      set pResponse.Name = person.Name
      set pResponse.LastName = person.LastName
      set pResponse.Sex = person.Sex
      set pResponse.Dob = person.Dob
      
   }catch{
       Set tSC="Error saving the person"
   }
   Quit tSC
}

XData MessageMap
{
<MapItems>
  <MapItem MessageType="WSTEST.Object.PersonSaveRequest">
    <Method>savePerson</Method>
  </MapItem>
</MapItems>
}

}

Well, I'll answer you from my experience, maybe I'm wrong.

About your first question, you are totally free to use a encoded BP or BPL and DTL, as you prefer. I usually prefer to use BP by code for custom objects and DTL and BPL when I work with HL7 messages...why? because I'm too lazy to write code foreach transformation of field in an HL7 message.

The Business Service used by the RecordMapper has a property called Synchronous Send:

When you check it the Business Service is going to send the record mapped in a sync call to your BP, so it's not going to read a new line of the file until a response is received from the destination.

About the OnRespone, you can check the official documentation: https://docs.intersystems.com/iris20231/csp/docbook/DocBook.UI.Page.cls…

I usually implement the OnResponse to receive Async responses from Business Operations called by my BP, in your case I think that it's unnecessary, but this is a personal opinion.

Take a look at this question of the community:

https://community.intersystems.com/post/how-copy-array-values-json-obje…

If your Business Operation is deployed in a Production you can configure the default options of EnsLib.HTTP.OutboundAdapter to control timeouts and retries:
https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cl…