Yes, there's a "raw" syntax, but I think it's counterproductive in the long run. Segments and fields can be addressed numerically, i.e. target.{1:4} would reference MSH:4. Not very descriptive; one of the beauties of using the DTL editor and message DocTypes is that your transformations become somewhat self-documenting.

You could attempt to build an HL7 "SuperSchema" DocType/Category, I suppose, if your intent is to address message elements using the "symbolic" Virtual Document syntax. For that, you need a DocType.

$E is the shorthand version of $EXTRACT ...

ConvertDateTime(source.{PID:DateTimeOfBirth},"%Y%m%d%H%M%S","%Y%m%d") should work just fine.$E(source.{PID:DateTimeOfBirth},1,8) by itself should provide the same result.

But ConvertDateTime($e(source.{PID:DateTimeofBirth},1,8)) is basically taking the 8-digit date returned from $E() and converting it without any input and output patterns ... which gets you the exact same 8-digit date.

The field definitions are properties of the *.Record class, so you could perform a property query against %Dictionary.Properties using the *.Record class as the parent.

SELECT Name
FROM %Dictionary.PropertyDefinition
WHERE parent = 'OSUMC.RecordMap.Patient.Record' AND Name >='A' AND Name <= 'z'
ORDER BY SequenceNumber ASC

That would get you the field names in the same order as the data and exclude any percent properties.

Or you could write a custom Business Process to do it. Here's an example with inadequate error processing (😉) that should give you some ideas:

/// Business Process to Modify the MSH:7 field
Class HICG.Sample.SetMSHDate Extends Ens.BusinessProcess [ ClassType = persistent ]
{
/// Downstream processes or operations to send messages to
Property TargetConfigNames As %String(MAXLEN = 1000);

Parameter SETTINGS = "TargetConfigNames:Basic:selector?multiSelect=1&context={Ens.ContextSearch/ProductionItems?targets=1&productionName=@productionId}";

/// Clone, modify and send the message downstream
Method OnRequest(pRequest As Ens.Request, Output pResponse As Ens.Response) As %Status
{
  Set tClone = pRequest.%ConstructClone()
  Set tCurDtTm = ##class(Ens.Rule.FunctionSet).CurrentDateTime("%Y%m%d%H%M%S")
  Do tClone.SetValueAt(tCurDtTm,"MSH:7")
  Do tClone.%Save()
  For i=1:1:$LENGTH(..TargetConfigNames,",") 
  {
    Set tSC = ..SendRequestAsync($PIECE(..TargetConfigNames,",",i),tClone,0)
    Return:$$$ISERR(tSC) tSC
  }
  Return $$$OK
}

/// Return an array of connections for drawing lines on the config diagram
ClassMethod OnGetConnections(Output pArray As %String, pItem As Ens.Config.Item)
{
    Do ##super(.pArray,pItem)
    If pItem.GetModifiedSetting("TargetConfigNames",.tValue) {
        For i=1:1:$LENGTH(tValue,",") { Set tOne=$ZSTRIP($P(tValue,",",i),"<>W")  Continue:""=tOne  Set pArray(tOne)="" }
    }
}
}

The reason you need to clone the inbound message is that Start-of-Session messages are immutable. You must clone them, modify the clone and send it.

The code block action in a DTL is for writing arbitrary ObjectScript, not Javascript. It's commonly used for for data manipulation that can't be satisfied by the methods available in the FunctionSet; for example, extracting and decoding a base64-encoded PDF from an OBX:5.5 field and writing it to a file. It can also be used to interact with globals to maintain state between invocations of the DTL, or perform a database lookup, or even write values to the default device that will display in the Test tool. Very useful for debugging.

I would not recommend using it for operations that could potentially block. There's no built-in mechanism for setting a timeout so use a BPL for those cases.

You'll need to generate your own XML schema by importing either an xsd or wsdl. Once you've done that, you'll be able to use the schema in your DTL by selecting the Source or Target class as EnsLib.EDI.XML.Document, with the Document Type set to the name of your imported XML.